Right I know how.
First you need to have the stage of the pet with the item on it. then, to edit the person's adoptable (add on the image) you need to do these steps.
1. Go to the admin page
www.tigerandlion.myadopts.com/admin.php (don't even try if your a general member)
2.click change adoptables
3. (only if you haven't done it yet) upload the image to wildlife paradise (like you were going to make an adoptable.)
4.go back to the change adoptables page and click edit or delete a user's adoptable! XD
5. type in the id of the pet. To find the ID go look on the persons profile or the link they gave you, for the pet they wanted to get the item or use search. then fill out the things to find the pets ID and copy ut and paste it in the box that says ID: .
6. now change the custom image to the image you chose. Then press submit changes
7. that should work. If you still don't undestand (because of my confusing way of speaking) then answer asap so I can make a pictue guide!