You can use GIMP to edit the unit cards (it won't open models).

On your download check if there is a setup application inside any of the rar files, if not:
I think some versions of GIMP might come without installer, try getting it from here: http://gimp-win.sourceforge.net/stable.html
you'd need GIMP for windows (assuming you are on windows) and the run-time thingy - they come as applications you just double click to install.

I just finally installed it from there - thought I should finally look at the program, it definitely opens tga's and looks like it has lot of other features. Err, you are going to need to find someone that's been using it for a while to answer any more complicated questions though
....and you will need the help files as well (at bottom of page I linked to)