Basic Steps

There are some simple things you should try before posting and asking for help.

Reverse Image Search
Either download the file and upload it to Google Image Search or if you're in chrome, just right click and select Search Google for this Image.

You could also try Tineye.

Watermarks
A ton of images have a watermark at the bottom which can be used to find the site where the video is from. You may have to sift through a ton of pages, but it's a start.

Other Identifying Sites
You can try Name that Pornstar's Reverse Image Search or Name that Pornstar.
There's also /r/PornId and /r/TipOfMyPenis and Karma Decay.

Advanced Search by Identifying Features (Pornstars Only)
IAFD Advanced Search and Freeones Advanced Search
Hit or miss, but search by tattoos or hair color or race.

Using Quotes in a Search Engine
You can use quotes to search for the exact phrase. So if there is a caption or a title or any text around the picture, try searching that in quotes. You may find other sites that have the same picture that have an identifying comment.

Also searching basic categories like blonde or lesbian or amateur + pics might narrow your search. It's going to be all about bulk sorting or luck if none of the other steps work.

Chances are if the picture is hot, someone else also wants to know where it came from.

I'm no expert at finding sources, but these are basic things everyone should do before asking for help. They're easy and fast. Put a little effort in, after all, that's what you're asking us to do.

Good stuff!

Hope peeps follow your advice.

I think this is really helpful advice... Thanks bird17man!