Go to Phonebook, then click on the contact's name, then on the "edit" that appears in the lower left hand corner. Scroll down to "picture" and then click on the "change" button. That will take you to the menus for the preloaded media files as well as any you may have downloaded. Find the one you want and click "select", which will take you back to your contact. Click "done" and you're all set.