|I think this is roughly what happened:|
The real picture was only ever stored in the email. The photo gallery is just a means to view and edit pictures (rather like shortcuts). It was therefore a shortcut that went to the bin but the actual picture remained in the email. After you deleted the email, all you restored from the bin was the shortcut. As there was no longer an email, then there was no picture for the shortcut to find.
That is one of the problems with the virtual world presented to us by modern operating systems. They can give a false impression about where the picture resides. If you had "saved" the picture from the email (to Pictures folder or whatever) it would still be there and also appear as a link in photo gallery.
It means of-course that there is no way to get back the picture unless recuva can restore the email back into the email program, as I said before.
To avoid this happening again you either have to keep the email, or Save the picture somewhere on the computer (outside of email). Picture files themselves don't actually reside in the photo gallery. If you check in photo gallery there is means provided to determine where the real picture file is located.
Always pop back and let us know the outcome - thanks