Okay, I really have no idea what's going on now. Just to test if ANY head would work, I took the head.tga file from the hampa folder and put it in my "test" folder. I typed
"/lp 0 test" and it worked.
However, I then opened the head.tga that I got from the hampa folder (the one that worked) in GIMP and added a single black dot to the picture (like a nose I guess). Then, I saved the file, without changing any of its attributes or its name. The only thing I changed was the picture itself, by adding the black dot. I opened Toribash again and typed "/lp 0 test" and the head didn't work!!
Seriously, the file was EXACTLY the same, it's just that the picture was slightly altered. The game KNOWS!!! It somehow knows the difference between the two identical files, one of which I changed slightly! It's like the game is actively preventing me from using my own textures! It's aliiiiive!! AHHHHHHHH!!!!
Seriously, does anybody know what the problem could be? I'm out of ideas.