Alright...everything has been pretty much said, but I'm going to do my best to clarify.
Step 1: Buy a head texture item from Torishop or Market.
Step 2: Have a texture design. You can make them using programs such as paint, photoshop, gimp, and other such programs. Or, you might find a free one in a forum, or you can buy one from another player.
Step 3: Go to your Deactivated items and find the head texture.
Step 4: Upload your head texture design to the item.
Step 5: Activate the head texture. (Steps 4 and 5 are interchangeable)
Step 6: Wear your new head texture, have fun, and enjoy Toribash.