Q: Can get my photos back once I have deleted them by mistake?

Photo stream is a service that syncs photos to other devices/computers.  What you should do is always copy the photos to another album on your computer, as the photo stream album is temporary.  Deleting them, as you have discovered, will remove them from photo stream, in time.  Your only hope at this point is to restore a backup of a device that had the photos in the Camera Roll - if you did a backup and the photos originally came from a device.  This assumes the photos would still be in the backup; if a newer one has been made since the delete, then I fear they are gone.