This is an example where you can find the textures of the archmage:
D:\Programme\Glest_3.1.2\techs\magitech\factions\magic\units\archmage\models\texture_archmage.tga
If you change texture_archmage.tga ( it's a picture ) you will see the changes in glest.
I think paint will not work. Most of us use the free gimp (
www.gimp.org ).
When you want to safe the modified picture in gimp press "save as" not "save". By this you can choose if the tga will be saved compressed.
It must be saved UNCOMPRESSED!! Glest doesn't support compressed tgas.