The game reduces detail and such as it mip maps the images to be smaller, take up less memory, etc. It should look better as you zoom in. There isn't much to be done about it other than increase the contrast on your skins. Other more graphically oriented people can feel free to correct me, since I don't really know what I'm talking about...
i usually get the skin looking good and then CRANK up the contrast and saturation. it may look rediculous in renx but it looks good ingame.
either that OR
if you are making a big mod then reduce the saturation/contrast of the textures already in generals. like the terrain and buildings and trees. then your structure will look cool.
This may be assuming to much or be completely wrong, but I think if you use a tga file the game will create its own mip map images and put them into memory where as a dds file already has mip maps built, which is unusual, but probably saves some time building mip maps.
Try saving it as dds first...you should see a difference ingame. Sometimes I work on a texture as tga...and it looks really messed up as tga in the w3d viewer. It seems kinda like the game was ment to use dds ingame for most textures.