When you're saying you're uploading them, you mean uploading to Battle.net, right? As screenshots for arcade section?
Actually that would explain something.. because I did notice such nothing for some other random arcade map.
Anyway, it's most likely a flaw in the way Blizzard is processing TGA files. Possibly your TGA images have embedded color map which SC2 can't handle. Or what's more likely it just strips the header that included the color map, without adjusting the image. What in the end may give such effect.
Your options are:
- use TGA without embedded color map
- or simply use JPGs.. as they're better suited for this use-case anyway