Thanks. The problem was not the cache. I'd cleared that. I didn't have any images of the size required for thumbnails, so I'd just uploaded content photos. When I checked page source, I found that if I hadn't uploaded a thumbnail, the image I uploaded to overwrite the previous content photo was saved as a thumbnail.
I have the image issue figured out now--except that I don't understand why an image larger than the stated dimensions can be uploaded as a thumbnail.