That is an interesting situation where a group of users might opt in and HTML could be a useful thing.

Are you sending photos via email? Aren't browsers generally a more pleasant way to view photos than mail clients?

Having the thumbnails show in the email itself means people don't have to break their stride to know whether the photos are ones they're interested in. I keep in touch with a lot of family and old friends using the same email. Different people care about different photos.