It's well known that the way GIMP saves indexed TGA files doesn't work well with ASH. But I've found a simple way to edit some (not all) of the indexed +1/+2 files without using Photoshop (since my 30-day trial ran out.)
1. Open the image in the GIMP.
2. Change any transparency to magenta, just like you would in Photoshop.
3. Make your edits.
4. When you're done editing, don't index it just yet. Save it as a regular RGB color TGA.
5. Get Irfanview (
http://www.irfanview.com/) and open your TGA in it. Go to Image>Decrease Color Depth.
6. Here, Irfanview can index your image for you. But here's the catch:
Irfanview can only save indexed images in 256, 16 or 2 colors, even if you put another number in the Custom box. This worked out well for Sonic's eyes - but not so well for CUPASH.
7. Go to File>Save and save over the old TGA.
8. Run step2_create.bat, of course!
I was very happy when I found this. I'm much more comfortable in the GIMP anyway, so I'll probably just use that from now on.
Sometimes this won't work, and ASH won't accept the file. If this happens, try using GIMP itself to index the image. If neither works, you've gotta use Photoshop.