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.