You can see one way I user personas; How to do a UX review and read about my mixed feelings about using them; Personas: the good the bad and the ugly.
Update: 1 May 2019 look!
Those lovely people at Creative Commons have released a search engine for royalty free photos:
Rob Whiting suggested a discussion thread on UXMastery: Where do you find persona images?
Mathieu Croset suggest UIFaces which shows user avatars mostly taken from Twitter so you might recognise a few faces there.
Mike Ellis shared his bookmarks of stock photos. As did Dan Goodwin and Mike Dunn.
Steffi recommended searching Flickr using advanced search and licence filtering.
Dan Goodwin also mentioned Jonathan Abbett’s persona photos collection.
Eat Everything offered his Bristol photos if the usage is for internal proposes. Ask his permission if you’d like to use them.
If you have any more suggestions please add them in the comments.