This is already working with a little trick since several years:
Go to ->Search ->Book Search ->Advanced
tick 'My Shelf Only'
And search for the title of the book, where you like to add a cover.
I tried this with two books now. Both registered by others, but in my hands. I found the books in my "all books"/in hands list, and then used another tab to search for those books. I could perfectly well add the picture, but in the "all books" list the pictures don't show up. But then, on the other hand, if I open the page for one of those two books, the photo shows up!! Weird.
ETA: now the covers show up both places..