Re: Attention sprite makers...
 

If you happen to be on a Mac... Graphic Converter is the program to use for image type conversion. It is also free I believe (at least it used to be).

I use Photoshop, but then, I am a graphic artist and have to have Photoshop to do my work. Most people would probably not want to drop the money for it.

All that said, I have never made a Mod (I am quite too lazy, I think), but I would be happy to lend my graphics knowledge to anyone who needs it.

Re: Attention sprite makers...
 

IrfanView is a much smaller program that can also convert .bmp to .tga.

However, Gimp has several nice features that you'll find useful, like layers (you can have several images that don't affect each other, like you drew on several pieces on transparent plastics placed on top of each other) and tools for changing colors in various ways.

You probably know about it already, but you can get the original sprites from:
Organized Unit Sprites. It's an awesome resource, because you can draw over existing units easily, and e.g. mounts are pretty much interchangeable between units.