Depends where they were - if they were on an SD card then you can quickly remove it (before anything gets overwritten) and then attach to a PC or laptop to use something like Piriform Recuva to search for the deleted files. Files aren't deleted just marked for re-use so if you're quick you can often get them back (the longer you leave it the higher chance of them being overwritten and lost) . Same may be possible on Android if you connect the phone via USB.
If it's an IPhone then unless you have them in ICloud backup it's not so easy...
https://discussions.apple.com/thread/4718118?start=15