For many authors, writing erotica free the easy part.
Erotica Stock Images
Other than those teens blowing dicks who are also photographers. I declare shenanigans. Being multi-talented is cheating.
There are some sites which offer stock images for free. Now owned by Getty Images, this site offers premium results first, but scroll down a little to find free images. This is the granddaddy of free image sites.
Still, if erotic digital image editing photoes are pretty sharp, this is a very handy resource. Prices vary widely, and again you tend to get what you pay for. There are authors who double as photographers, sneaking off to their BatCave at 3am with sexy-as-hell models and snapping absolutely flawless shots because, as writers, they already know what you are looking for.