When you sell a game you should hand any copies or backups with it as well. In most Western countries ( I don't know much about copyright laws outside of those ) you might be allowed to have a backup of a piece of software, only if you are in the possession of the original. Selling the original takes away your right of having a backup as well.
Besides, downloading (non free/share ware) software from the internet is illegal anyway, as technically you're only allowed to have a backup of your own copy. Not someone else's. I know that the data is the exact same, but I think that's what the laws and agreements often come down to.
Hence why here at NGEmu we don't offer support for downloaded images (1st reason), also because when it's from the internet the source can be rather unreliable, there's a lot of bad images floating around causing problems. (2nd reason)