This ends up being too big for livejournal, but the technique was requested. This was made in GIMP, so the last two steps (how to make an animation, very very briefly) are GIMP specific. The rest of it shouldn't be.
from
and
1. I started with these two pictures that I wanted to fade between:
A:
B:
(I can explain how I got the effect in a different tutorial.)
2. I made copies of both pictures and put A above B. I changed A's opacity to 70% and merge A down. (I decided to use 70% and 40% because it gave me the best transitions in the least number of steps. Your own opacity percentages may vary.)
70A:
3. I actually copied 70A and put it above another copy of B. I set 70A to 40% opacity. (I found this easier than copying both A and B, and yes I know the exact percentages of A and B in each frame are actually different than I stated. Shush.) Again, I'm not changing any modes, it's all Normal. Merge 70A down into B to get 40A.
40A:
4. Now I made a copy of 70A and 40A, and arranged the layers like so:
40A
70A
A
70A
40A
B
5. Then I went to Filters->Animation->Optimize for GIF and tweaked the times (the number before ms in the name of each layer) to get something I was happy with.
6. I made sure to save the resulting file as an animation.