FIXED! It appears that my replacement of the background image has worked after all, it just took a little while for the changes to appear.
Thanks everyone!
Marking as resolved.
Stef415 (or anyone else), could you be a bit more specific on the process for this? For some reason this didn’t change the size of my cart images (I also regenerated my thumbnails to the new size). Surely there’s a way to designate cart thumbnail size through the cart.php?
Thanks!