If you're using one of the stock wallpapers I don't know of a way to do it, but I don't see why you'd want to hide any of those, as they're pretty innocuous :)

If it's an image in a user's home directory, you could try changing it so it can only be read by that particular user: right-click on the file, select Properties, then Permissions, and set "Access" for both Group and World to "None". The login manager should then be unable to read the image and will show a default image instead.