I thought I might have been in a wrong set before. I had originally started the units from 2100 going up, and the same file encountered the problem, when it was 2103 and now 2003. I'll try moving the files up to 2200s and see what happens.
I always save my Gimp TGAs in a folder, and then use IrfanView to open and resave them to the mod folder. I'm a bit hazy on the details, but I think Dominions had some trouble with the spesific type of tga files GIMP saved a year or two ago, when I started doing this.
I'd actually already dug up an old post you made about that and tried it. While I have discovered that I like IrFanView more, unfortunately it still isn't getting the files to work.
I use GIMP with no problems. Sometimes (generally when I've been using layers), Dominions for some reason shows all the black and pink instead of making it transparent. When that happens I have to copy and paste the sprite into a new image, then it's fine. However, I've never had a problem like you describe. Maybe try posting one of your sprites, and we'll see if we can reproduce the issue.