so good a name they named you twice !
Adding an image is certainly tricky, I have tried without success recently.
But wallpaper’s a good idea.
Adding a working URL may send your post to be checked by admin and a long wait, and you may think that you made a mistake and then try again with the same result.
However, you could add a link to dropbox by leaving a space or spaces in the link, but you must check your post immediately after as you only get a very brief time to edit posts, maybe five minutes at the most.