Thanks for your reply. The images are not coming from a camera or other external device. Most of them were copied to the computer from a dvd a friend burned for me. Others were downloaded from the Internet, scanned, etc.
The images vary in size. Some are medium (400 x 600 pixel range) while others are quite large.
I've done virus/malware scans with multiple applications (AVG, Avast, Malwarebytes, Superantispyware) and am coming up clean. When you mentioned the possibility of drive errors I realized that only the images on my slave drive were affected. I scanned the disk, and it did indeed find errors and corrected them. This is not my OS drive, which is why there haven't been OS errors as you described.
Thanks for the tip. Hopefully this has resolved the problem. At first glance it appears to have fixed it.