One more thing… in the above code for displaying the PrettyPhotos, I found and fixed one more issue.
As I’d mentioned, I noticed that the lightbox was working. Then I looked at its own little navigation thumbnails at the bottom. There were tiny thumbnails for the Posts with Featured Images, and also “blank” thumbnails for the “other images”. When I clicked the “other images”, a lightbox error came up saying “image not found”.
I realized what was going on, and then made the lightbox code conditional – I just used an IF statement to make sure there was an existing featured image before adding a mini-thumbnail. This way, I only got mini-thumbnails on the lightbox for “real images”. 🙂