Gimp is an art program, like paint. Which program did you use to make your head?

I think that you can also upload a png or jpg,
it doesn't have to be a tga file anymore.

Yep, thats true but if you just create a folder in the custom directory and put your head in there and it isnt a .tga, it wont work. If its not a .tga, you must upload it through the "textures" link in the User Cp.

But if he'll just put it under the custom directory, only he will see his head texture, but others won't.