There is no such thing as free porn pictures to use on your websites. Where you buy your licensed content will probably depend on how much you want to buy and what type. There are sites that give you the option to buy a single picture at a time and others that dont and only allow you to buy complete "sets" of content. I recommend you check out the GG & Jims resources section. It has a nice list of content providers there.
http://www.greenguysboard.com/resources/content.html
When shopping for content its always good to shop around. Theres almost a 100% chance that someone will have exactly what youre looking for.
Cheers and welcome to GG & Jims!
WB