Sponsored tweets but keeping the copy along the same lines. Also I see some tweets where there could be an affiliate link attached to it.

Hmm, I'd be interested to see how well sponsored tweets would work.
I would worry that it may ruin their artistic / indie vibe.

Obviously the key here is to: "Subtly sell to the audience which you've already created"

One option may be to throw ANOTHER product out there of the back of their twitter following.

For example:

a) They could announce a monthly photography competition with prizes and a submission fee to enter. They could use their twitter feed to feed to drive people to their competition and then promote the work of the winners on their twitter feed.

b) Get in contact with a publisher. Tell them you have a massive following and see if they would put money behind you creating a photography book. Then they could sell the book to their audience.