I responded to a journal post last week where an artist was asking about free references -- some people found it helpful, so I thought I'd post it here.

Awhile back, I compiled a list of copyright free image sources:

www.pixabay.com (Creative Commons Public Domain CC0)
www.picjumbo.com (Photos themselves are not for resale,
anything you make with them is)
www.morguefile.com (Commercial reuse with modification)
www.littlevisuals.co (Creative Commons Public Domain CC0)
www.thisisamericafoundation.org (Donated to the public domain)
www.picography.co (Creative Commons Public Domain CC0)
www.getrefe.com/downloads/category/free (royalty-free license use in any project for unlimited times. You can not, transfer the licence or resell the photos.)
www.jaymantri.com (Creative Commons Public Domain CC0)
www.publicdomainarchive.com (Public Domain)
www.goodfreephotos.com (Public Domain)
www.unsplash.com (Creative Commons Public Domain CC0)
www.mmtstock.com (Creative Commons Public Domain CC0)
www.pexels.com (Creative Commons Public Domain CC0)
www.thestocks.im (This is a compliation site of many of the other sites listed here. Creative Commons Public Domain CC0)

There's also New Old Stock: https://nos.twnsnd.co/ But they don't know if the images are copyrighted or not, so that's more for reference/inspiration