You have to upload the Art into the head texture.
Click the shop tab, then scroll down. On the left side you should see a little link that says "Textures". One of the first slots should be for 128x128 heads. It will have a bar that says Browse, and Upload.
Click browse. Find the art file (.jpg / .png / you get the idea) and then upload it.
Next time you go ingame you should be wearing your new head. Enjoy!