Once upon a moon, when I ran some forums. There was some server based software that would allow images to be uploaded "anysize" to the server by users.
It would resize and compress the images for maximum viewing pleasure vs minimum storage size for the image database. The complete image database could be saved in one directory as a .zip format file for easy back-up.
Was probably a MySQL database. Also I would have though there should be a plugin from vBulletin itself or a 3rd party plugin?

Been awhile since I did this though.