Jim is having problems with Windows recognizing his external USB drive. But his image catalog says his images are there. Leo says that many photo gallery apps keep a thumbnail for fast referral. So it could have the thumbnail, but not see the original image, if the drive is disconnected or lost. Leo also says that his external drive could be getting flakey. He should get a backup drive and make a copy of his photos. He should save them online, too. Three copies, on two formats, with one off-site. The good news is that hard drives are cheap now. He can get a 1 TB drive for under $100. Scooter X says that OfficeMax had a 2TB WD USB My Passport drive for $69.99.
Another issue could be that Windows is losing track of the gallery because it's an external USB drive that comes and goes. So it could be losing connections. He should unplug it and plug it back in. He could also try rebooting.