Do you have enough free space left on the phone?
Did you already try to switch off the phone and start it up again after that?
Are those photos part of the camera roll, or have the pictures been synced via iTunes or Photo Stream?
Yes I have memory left on the phone. The photos have all been taken on the phone itself, so no synched ones, and yes the phone has been turned on and of many times. It's been like this for weeks now. :/
Force restart the phone by holding the sleep and home button for about 10sec, until the Apple logo comes back again. You will not lose data.
In case this does not work, back up the phone, set it back to factory settings and restore from the latest backup again.
Use iTunes to restore your iOS device to factory settings - Apple Support
Camera Roll Blank Tiles