Not just FB and Pinterest, but also G+, Delicious and many other bookmarking sites! Delicious used to work with almost 100% reliability, but it never picks up my opening images now!
For Pinterest, I find it much better to use my Pin bookmarklet, and select the photo/image I want to represent the hub - the first image, is not necessarily the best to pin.
Low and wide images are a problem for many sites. Even the gallery view here doesn't view all of the low-and-wide images, typically used as horizontal rules and opening banner photos.
Other problems that I've encountered are more intermittent - sometimes it works, sometimes it doesn't.
- Posting too soon after publishing can 'miss' the opening image.
- Referral links can cause problems for sites to automatically grab a photo.
- Very tall, or very wide photos may be ignored, as they violate the square thumbnail format.
I shall have to try Susan's short URL trick, as I'd given up with getting my photos to show.