When you submit an artwork, during the upload process, you'll see a box labeled "This submission was made for:" This is the gift box.

Click on the arrow on that box and a list of all your friends will show up. Choose the friend you want the gift to go to, and after you've fully submitted your work, it will also show up in your friends gallery under a special gift tab.

NOTE: You must be friends (you are watching them, they are watching you), in order for their name to show up in the gift box.