Simples, Flounder! You can't actually edit in the Gallery. You can only edit it in your Editor.
What you need to do is:
1. Find your art in the Gallery, and click on the "Remove" button underneath your profile picture to remove it
2. Go back to your "My Pets" page and click on the blue paint pallet underneath the art you want to change
3. Make your changes to the art
4. Save your changed art
5. Go back to your "My Pets" page and resubmit it to the Gallery.