What do you need.
-Gimp/Photoshop
-DDS plugin
1. First you delete or move the picture pinup.dds in folder data/textures/TNormal/tex/pinup.dds.
2. Than you make a new picture in Gimp and save it in /data/textures/TNormal/tex. And rename it in pinup.dds.
But importent is make an alpha channel and make it black.(100% transparent)
Or make the alpha channel and mix it with your picture.(alpha to color)
3. Now you can start the game and see your girfriend or your mother.
Example.
http://gallery.me.com/michal788#100044