Go into your Toribash directory, which is most likely to be C:/ProgramFiles/Toribash-2.8 or something similar ^_^
Now click on Custom, now make a folder called "gamer" then put your head in there, make sure it is a TGA (.tga) and name it head.
So you should now have a folder called "gamer" with a file in it called head.tga
Open up Toribash 2.8 and make your name gamer (in the setup).
Should work when you open up single player now.
Is the image already a .tga or is it Gif, or what?
Here is a link to download a converter, if needed: http://www.imageconverterplus.com/download/