i don't think payment really needs to come in at this point. but if one party is using them for profit without the others consent, then i think there are definitely some questions about ownership in regards to intellectual property and licensing and the like 

tnuts420

it doesn't have to be payment, just something of value exchanged. They both get the prints and the labor of the other. That'll do.

It's been a long number of years since my blaw 101 course, but wouldn't there be the need for "consideration"?   Even a payment of 1 sat would do.

cointastical

No need for payment but for a prior agreement

topsecret

Daily discussion thread

saloon

Let me present you a thought experiment:

Two people, fresh from school,  want to build their portfolio.

An amateur photographer meets with an amateur model.

The model poses, and the photographer takes some photos and sends them to the model.

Both keep the photos as proof of their work for their respective portfolios.

Who pays who?

Is there any need for payment in a situation like this?