Hi Dan,
I appreciate your reply.
New - Launched about 2-3 weeks ago. But already receives decent traffic and is crawled by Google every day. When I publish a new post usually in a few hours it appears in webmaster tools as submitted but Google simply won't index those images. Those which are already indexed are from the first post published but from then no new images are indexed.
And yes images are not unique. But I don't think it should be problem for indexing. Do you think they will index them later as you are telling "google may not prioritize indexing" ?
Thanks.