To add onto the above statement you can also make it a .BMP .GIF .JPG .TGA or a .TIF and upload it
here.
To change the file format of an image open it in any image software, click save as, then change the file extension to one of the above.
You can also preview textures without buying the head texture item, by following these instructions:
Save it as a head.tga and turn off REL compression.
Make a folder in you custom folder, for example "Head_Test".
Put the head.tga in that custom folder.
Go ingame and type /lp 0 "Head_Test"
Only you'll be able to see it unless you buy the head texture item and upload it.