You can add a photo to a sighting on the Add a Sighting page but at only at the time you originally enter the sighting. (I guess you knew that Steve as you have already added photos)
We are having problems with the Edit Sightings page which is where you should be able to add extra photos. For full details of the known problems here, see our reply to Steve Gantlett (dated 10 April) in the Teething Issues thread below.