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DID YOU LISTEN TO DRAKHIR?
Fine, if you're so impudent to not listen to his advice, here we go...
First, get an image. Any image. It could be bmp, jpeg, gif, png... whatever, so long as you can edit it.
Next, edit it as you want, and save it. When saving note these fundamental facts;
It MUST be 128x128 pixels. If you don't know what a pixel is, google it.
It MUST be a compatible filetype. This can be bmp, jpg, gif or tga. I would have you note, taking a head and, using Explorer or Paint to save it as 'head.tga' WILL NOT WORK.
Make sure that in your FORUM PROFILE, you have your In Game Name set as the name you use to play Toribash (this is in your User CP, where you entered your key).
Finally, upload the image you want as a head through the control panel. Don't ask where it is, it doesn't take even a minute to search.
Fianlly, wait for the image to be approved. I imagine it would be within 24 hours time, but it may take longer, and you may recieve an email saying that your head has been added.
If you are confused about ANY of that, don't complain. Search the forums... You could find ALL of this information in the FAQ section.