1. If you are wanting to have a perfectly round sphere at the end of this, you need a square image, otherwise, it doesn't matter. Whatever size you choose, however, you will need some "room" for the destruct effect. So figure out what size square you want, then add 50 pixels to one dimension. In this example, I want to end up with a 250 x 250 square, so I will make mine 300w x 250h.
2. Activate your Rectangular Selection Tool. In the window that comes up underneath the main Toolbox Window, there are some options for this tool. There is a drop down menu that has an entry for "Fixed Size." (The default is "Free Select") When you choose this option, the two number boxes below become active. Put in the values 250 and 250. Place your cursor in the upper left hand corner of your image at 0,0 and start to drag out a selection. Look what happens! Instantly a 250x250 selection is created with its upper left corner located where you started to drag. Leave your settings at Fixed for now, but don't forget to change them back, or you will be wondering what happened to your select tool the next time you fire up Gimp.
Do Select>Save to channel. We will be recalling this selection later.
3. Create a new transparent layer, select it and do Filters>Render>Clouds>Plasma, with whatever settings look good to you. Make it somewhat turbulent, due to the next step.
4. With the selection active, do Filters>Map>Make Seamless.
5. Do Select>Invert. Now you have a selection that is just the empty 50 pixel wide strip at the edge of your canvas.
6. Either double click on the plasma box layer in the Layers dialog or right click on it and click Edit Layer Attributes. Either way will enable you to rename the layer. Name it "Frame #1".
7. Duplicate that layer. Notice how the "Frame #1" has become "Frame #2." Keep duplicating frames until you get to "Frame #25". Create a new transparent layer above "Frame #25. At this point you should have a layerstack that at the very bottom, has the default white background layer, then 25 sequentially numbered plasma box layers, and at the top, one transparent layer with nothing on it.
8. Turn OFF all the layers except Frame #1, "Frame #2", and the background layer (turning them off isn't strictly necessary, but it will make it a LOT easier to see what you are doing and will help you to keep track of the changes you are making in each layer. Tip: If you shift-click on the eye icon, it will turn off all the layers, then you can go back and just turn on the ones you want. Shift click will turn on/off all layers at once, or link/unlink all layers at once. (Thanks to saulgoode for this tip.)
9. Starting on Frame #2, do Select>Grow and grow your selection by 10 pixels. Why not use the original selection and use shrink? Because that would shrink the selection uniformly in all directions, and it wouldn't work for what we are doing. If we use the 50 pixel wide strip and grow it, it can grow in only one direction, and that is across our plasma box, it is stopped from growing in any other direction by the edge of the canvas.
So now we have a selection that covers 50 pixels of empty space and 10 pixels of the edge of our plasma.
VERY IMPORTANT, LEAVE YOUR SELECTION ACTIVE! We are using the same selection to make multiple cuts on multiple layers, but growing it larger each time. This step is very repetitive and it may be easy to lose track of where you are and cut a layer wrong. Take your time, verify that you have cut each layer correctly before moving on to the next. Each succeeding layer should have a plasma box that is 10 pixels narrower than the one underneath.
Grow your selection by another 10 pixels and press
From here just work your way up the layer stack, shaving 10 pixels more off each succeeding layer until you get to layer 25. Turning layers on and off as you work your way up. After each cut, you should always see your selection overlapping a ten pixel wide strip of the layer underneath the one you just cut. If you don't see that, either you cut something wrong (undo is your best bud, here), or you haven't been turning the layers on and off correctly. This will help you keep track of what you are doing. What you are doing is sort of "leapfrogging" your way up the layer stack as you turn your layers on and off and select them.
For starters, you would have Frame #1 and Frame #2 on. You would cut 10 pixels off the edge of #2, but you would see the ten pixels of #1 below. Now you turn OFF #1 and turn ON #3 and cut it, but you see the ten pixels of #2 below, and so on.
11. We now have a layerstack with a series of progessively larger plasma boxes, i.e., no box at all on the top layer, the layer named Frame 25 has a 10 pixel wide strip of the box, the next 20 pixels wide, and so on, until we get to the last layer, which is the full 250 x 250 pixel box filled with plasma.
Now we are going to make the destruct.
12. Turn off all layers except the background and frame #2. Select Frame #2. Either do Layer>Transparency>Alpha to Selection or right click on the layer in the Layers dialog and click Alpha To Selection.
Do Select>Invert. Grow the selection by 10 pixels once again, and again, we are overlapping a 10 pixel wide strip of our box.
13. Now do Filters>Noise>Spread with a value of 50. I like to run the filter twice, but you don't have to.
14. Now work your way up the layerstack, growing your selection by 10 each time and running the Spread filter. What we are doing is "spreading" the edge of each of our layers to make it look like it is disintegrating. This time, I would recommend just turning on the layer you are working on, plus the background (its easier to see the spread), with all other layers off.
15. Delete the default white background layer.
16. Save your file first as an .xcf. Then do File>Save a Copy and save as .gif. When the first GIF save dialog pops up, click "animation" and ok, when the second one pops up, set your timing however you want and then make sure you choose "replace" mode for the frames. You will have saved what I will call a Linear Destruct File. The idea of this has possibilities in and of itself. If you were to reverse the order of the frames, you would "create" rather than destroy. This would be an interesting technique for animating text for a variety of purposes.
17. Now we still have our original .xcf file. Save it again, but this time call it "sphere-destruct.xcf", that way we start a new file we can work with, but we still keep our original .xcf file to do other things with.
18. Now we are gonna make the sphere-destruct. First, do Image>Transform>Rotate 90 CW, so that the edge with the spread is turned toward the bottom of your monitor.
19. Remember that Selection we saved? Now we are going to recall it. Click on the channels tab (In the default Gimp configuration, it should be the one just to the right of the Layers tab.) There will be four regular channels, for each RGB color, plus alpha. The Channel we want is called something like "Saved Selection". Right click on that channel and do Channel to Selection.
20. Click the Layers tab to get back to the Layers and then click on the layer labeled "Frame #1" to select it. Do Image>Crop Image, the 50 pixel wide transparent section at the bottom of the canvas should go byebye.
21. VERY IMPORTANT: Do Image>Transform>Flip Vertically so that the "spread edge" is now toward the top of your monitor.
21. Starting at the Frame #1 layer, work your way up the layer stack and run Filter>Map>Map Object with the following settings (or your own settings if you want). You won't have to put the settings in every time, they will stay the same by themselves unless you change them. Map each layer to a sphere. Yes, some of them will seem "weird" since there isn't much on them except blank canvas. Don't bother to map the top layer, since it doesn't have anything at all on it. If you want to save yourself some hassle, you might set up a dynamic keyboard shortcut so you don't have to click through the menus every time.
Another thing to note, as you are doing the sphere mapping, it may seem to you that it isn't working right. If you are turning each layer on as you go, you will be seeing the cumulative effect of all layers. When you go to animate, it won't look like that, however.
22. Save your file again, first as an .xcf, and second as a .gif. Use a different filename, of course, but use the same settings for your animation as you did last time, at least for the "replace" mode, anyway.