The first thing to do is to download and install the Plasma 2 filter. It is available for both Linux and Windows here.
Start with a fairly large canvas, at least 600 x 600 (square keeps things from getting "mashed" too much in one direction or the other when we use the perspective tool.
The installed Plasma 2 Filter will be found in Filters>Render>Clouds>Plasma 2. You need to make one change to the filter settings and maybe click on new seed. Change the drop down menu to "Cauchy". This gives you a some what "lumpier" looking plasma than the regular plasma filter.
Now, from the menus across the top of the canvas, do Layer>Colors>Desaturate.
Then do Filters>Map>Bump Map and bump map the sheet of plasma to itself (the source image will be in the dropdown menu in the bump map dialog). Fairly simple to do, if this is the only image you have open. When you do this, pay attention to where the light is coming from in the preview window and adjust the azimuth setting accordingly. This is kind of how one face of your mountain will be "lit". Adjust the other settings to give a nice "rocky" look to the plasma. Below are the settings I used for this tutorial.
Now, duplicate the plasma layer and turn both layers off. The reason we turn the layers off is that if they are on, you see the actual layer you are deforming in its undeformed state, plus an overlay of it in the deformed state. You have no way to see how the second half will line up with the first if you don't turn off the layer by clicking the "eye" icon for it in the layers dialog. The layer will be "invisible", until you start to use the perspective tool on it. Once you click in the canvas, the transform handles will appear at the corners and when you start to move them, the deformed layer will be visible against the checkered "alpha" background.
Optional: At this step, if you want, you can make a whole different second plasma layer if you want. This might give more variety, however the next steps will mix up the existing layer pretty good, so this may not be necessary.
Select the bottom layer. By default, it has no alpha channel (the ability to have transparency), the second layer has an alpha channel. We are going to use the perspective tool on both layers to make two "faces" of our mountain. The perspective tool will automatically add an alpha channel to the bottom layer, so don't worry about having to add one.
Use the perspective tool to shape your bottom plasma layer into something like the screenshot below. You will want to use the zoom level drop down menu at the bottom of your canvas to zoom things out a bit so you can see the handles of the perspective tool more easily.
Once you have things deformed to your liking, click "Transform" in the perspective dialog (watch out, it may be hiding behind other windows, check your taskbar), to complete the transformation.
Turn the layer back on so you can see this half of your mountain and so it can be used as a reference for lining up the other half of your mountain.
Select your second layer and use the perspective tool on the second plasma layer and deform it so that you get a second "mountain half". Make sure it overlaps the first mountain half by a bit so that you don't have to go in and clean any gaps up with the clone tool later.
Once you have both your mountain layers, right click on the top one in the layers dialog and pick "Merge Down". You may well have a dotted yellow line across your canvas at this point. This is the layer boundary. If you were to save out the image at this point as a jpg or png, that line wouldn't be there, it just shows where the edge of that layer is. Lets get rid of it and make the layer the same size as the canvas. Again in the layers dialog, right click on the newly merged layer and pick "layer to Image size". Presto, the line is gone.
Save your file as an .xcf at this point. (if you don't know what .xcf is, it is Gimp's native image format. Saving in .xcf will preserve all your layers, channels, paths, etc. in case you want to go back later and change something. When you want to save as jpg, or png, then do File>Save a Copy, and save in your destination format.)
Now let's sculpt our mountain.
Do Filters>Distorts>Iwarp. The default should be "Move" mode. Jack up the deform radius to around 40 or so or even higher. Move your mouse around in the preview window to sculpt the shape of your mountain.
Don't get too wild, or your "virtual rock" will get stretched too much and make things look unnatural. This is part of the advantage of working with a larger canvas, there are more pixels to work with in Iwarp and it helps to minimize the effects of the distortion. The main thing in this step is to break up the straight lines and "regularness" of the mountain shape. Do not only the outside outline, but the straight "join line" down the middle where your two halves meet. You want to get something that looks like this:
Now the line down the middle is pretty crisp looking, even though we have distorted it. There may also be some alpha showing through. We don't want either of those things. So with a large fuzzy brush (you may want to make one in the brush editor), pick the smudge tool and sort of randomly smudge on that crisp line. Again, not too much (maybe even lower the opacity/rate of your smudge brush in the tool dialog) or it will look unnatural. You may also need to use the clone tool to clean up any gaps between the halves. Putting a temporary white or perhaps red background behind the mountain layer may be helpful to see if there are any gaps.
Now lets add a shadow to the right side of the mountain.
Create a new layer above your mountain layer. Get the lasso tool and make a selection that looks like the one below. Don't worry about being too precise, but you do want to roughly follow the contour of the line down between the two mountain halves. Here I have put a temporary white background behind things so you can see them better.
Once you have your selection, turn it into a quickmask. You can do this one of three ways. Down in the lower left corner of your canvas is an icon. Clicking it toggles quickmask on and off. You can also do Select>Toggle Quickmask, or press
When you turn on quickmask, your screen will get a red overlay, except where your selection is. This red overlay is actually a temporary channel (a black on white image which is visible in your channels tab). You will notice in your layers dialog that your layer is no longer selected. if you click on the channels tab (the one to the right of the layers tab in the layers dialog window), you will be looking at your channels.
You can do things to this quickmask. it behaves just like a regular image. If you paint on it in black in the canvas, it will appear to paint in red. If you paint in white, it "erases" the red, revealing more of your image. You can run filters on this quickmask. What we are going to do now is to run Filters>Blur>Gaussian Blur on our quickmask.
Okay...our overlay is red, but our blur shows black on white (you may have to scroll around in the preview window a bit to see this). This is because that is what a channel is, a black and white image. Find an edge of your selection in the preview of the Gaussian Blur window and set the blur at about 6-65 or so, then click okay.
Your quickmask should now be blurred like this:
Toggle the quickmask off to go back to a selection.
What did we just do? We feathered the selection. Why do it that way instead of Selection>Feather selection? Because by using the quickmask, we can see beforehand how our feathering is gonna look. If you just do Select>Feather, plug in a value and run the filter...how do you know what the effect is gonna look like? You may have to do an "Undo" and spend a buncha time fiddling around with the settings. Blurring a quickmask lets you sort of preview the feathering of a selection beforehand.
So now we have this feathered selection (but we can's see it). On our new transparent layer, bucket fill the selection with black. Cancel the selection. (Select>None).
In the layers dialog, with the layer we just filled selected, set it's mode to multiply and adjust the opacity slider to give the mountain a nice shadow on one of its faces.
Merge the shadow layer down onto the mountain layer.
Basically, our mountain is done.
But lets add a cloud layer behind the mountain.
Make a new transparent layer and move it to the bottom of the layerstack so that it is underneath your mountain.
Do Filters>Render>Clouds> Plasma (regular Plasma this time, not Plasma 2 -- though you could probably use it with some changes).
On your newly created Plasma layer, do Layer>Colors>Desaturate.
Now do select by color, either from the Toolbox or Select>Select by color. Pick an area of color on your plasma sheet.
Once you have a selection, do Edit>Cut or press
Now use the perspective tool to make some clouds in perspective behind your mountain, like this:
Again notice that we have zoomed out to be able to see the handles of the perspective tool, especially since we are applying quite heavy distortion to the layer.
This also has some added layers to color the clouds. I had to do a bit of cloning/smudging down the seam in the middle of my mountain where I didn't have things lined up as well as I might have and the cloud layer showed through.
Anyway. You now have your very own mountain.
Other possibilites: Use more than one sheet of plasma stitched together to make more crags/valleys. You might even color the sections and blend them together somehow. Or how about using multiple sheets and somehow sculpt/stich them together with Iwarp and maybe a touch of cloning? Or how about adding snow? Trees? I will leave it to you to figure out how to do those things.