The textures for the underwear are in the folder "Texture" of your singles directory. The textur which is used at the moment is named slip_weiss.png. There are different underwear types in this folder so if you want to have another you just have to rename the file with the underwear you like in slip_weiss.png. Before you should make a backup of the original file slip_weiss.png (i.e. rename it in slip_weiss.png.bak).
Below the pictures is always the name of the file which contains the underwear (this file you have to rename in slip_weiss.png).
If the game shows the old underwear after you made the changes you have to open the modified file slip_weiss.png with a graphic progamm (i.e. Paint Shop Pro) and then save it. So the date will be updated and the game takes this new texture.