1. Personal relation: if this pic from someone you know (even virtually) it's more interesting
2. Image quality: pictures that look good or more professional are more interesting. Blurred pics don't get my attention.
3. Originality: another dick pic or shaved pussy is dull, but hair, tattoos, humor, public location, interesting clothing, lingerie etc. will make people more curious.
4. Personality: pics that say something about the person are more interesting than pics that only show his/her organs. Things like ethnic clothing, books, bedroom, environment etc. will tell a story and attract more interesting viewers.