It's not a "gentleman's agreement", it's promotional material.
The porn sites themselves build the picture sets and if you have an affiliate account with them you see the available material (along with all the relevant html code that you can customize to match your colour scheme or whatnot and that already has your referral codes in place) and pick what you want to use on your own site.
The porn sites themselves build the picture sets and if you have an affiliate account with them you see the available material (along with all the relevant html code that you can customize to match your colour scheme or whatnot and that already has your referral codes in place) and pick what you want to use on your own site.
It's basic affiliate marketing.