Once you deleted your own local copy, the folder of photos on Dropbox was no longer a "backup." You just moved your one copy elsewhere. You essentially had NO backup. And everyone should know, you should always have at least two backups. (Again, you had zero.) I wouldn't even consider Dropbox (without Packrat) a valid backup destination either, because any changes get synced.

Sorry this happened to you, but better backup practices could have avoided it easily.