This tutorial is fairly simple, just time consuming. The result will end up something like this :

Ok first of all be happy, their GAP is not required. ^_^
There are two ways to do this tut, the fast but not as good but still good transition way, and the slow but slightly better transistion way. Either way is fine. The only other difference is that one takes more frames than the other, twice as much.
I apologize that this doesnt have a better explination with images, but i hand wrote this last night and am posting this up at school ^^; I may go and get some Screen shots later and put them in to make it somewhat clearer.
Ok now we get to the tut.
1. Open the two images you want to transition. Make sure they are the Same size. Otherwise it wont work.
2. Open a new file that is the same size as the two images.
3. Take one image and paste it to the background.
4. Now duplicate it twice and hide the top one. (Click the eye in layers).
5. Now creat a new layer under the second duplicated layer and paste the other image in.
6. Decision time! This is where you decide whether to take the long path or the short path. Either way they get the job done. the fast way is to set the opacity to 4%. The long way is to set the opacity to 2%. If you are impatient/lazy (like me) I suggest you use 4%.
7. Now merge that layer down to the duplicated layer under it.
8. Now duplicate the unmerged duplicated layer (the hidden one at the top) and make it visiable.
9.Create a new layer and paste the second image again.
10. This time we will be adding +whatever % opacity you choose to what it was last (This is where it gets repetitive.) 4%>8%>12%>16% etc. 2%>4%>6%>8% Etc.
11. Now repeat steps 7-10 until you reach 100% opacity (when it has completely turned into the second image).
12. Ok good job ^-^ We've gotten one image to fade into the other, but we still have to go the other way o_O"
13. Now the return process has 3 ways of doing it.
13.1)Duplicate everylayer and place it so its backwards
13.2)Do the same process just backwards (100%>96%/98%>92%/96% etc.)
13.3)Do it the same way (starting with step 4)
13.4)Remember, the opacity *MUST*
14. Almost done! Go to Filters>Animation>Optimize for .gif
15. Set the two plain images at 2000 MS(if you want it to stop that is.) If you dont want 2000 MS any other would work, just keep choosing what you want and keep playbacking it (Filters>Animation>Playback) until you get something you like
16. Leave the rest of the frames at 100MS.
17. Now we save, you do know how to save an animation right? File>Save as>.Gif Image> Save as animation.
18. And its done! ^_^
comments on the failureness?

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