Photo credit goes to FilterGrade at https://unsplash.com/@filtergrade
These best sites I could find for quality, hi-res, artistic images for free. Please always remember to check the rules and licenses for each website as they may change without notice. Also some of these sites might not require credit, but it's always nice to do so.
Hi-Res Artistic
https://unsplash.com/ - Free collection of artistic hi-res photos. I check this site every week just to browse the new photos.
http://gratisography.com/ - "Free high-resolution pictures you can use on your personal and commercial projects."
http://nos.twnsnd.co/ - New Old Stock has "vintage photos from the public archives free of known copyright restrictions."
https://picjumbo.com/ - A free private collection of hi-res photos done really well.
http://getrefe.tumblr.com/ - "Refe is a marketplace of royalty-free, high quality, natural looking photos of people interacting with technology. Here are free photos for your personal and commercial projects."
Stock Photos
https://pixabay.com/ - "Finding free images of high quality is a tedious task- due to copyright issues, attribution requirements, or simply the lack of quality. This inspired us to create Pixabay - a repository for stunning public domain pictures. Your source for free vectors, free drawings and free photos. You can freely use any Pixabay image without attribution in digital and printed form, even for commercial applications."
https://morguefile.com/ - "This morgue file contains free high resolution digital stock photographs and reference images for either corporate or public use. The purpose of this site is to provide free image reference material for illustrators, comic book artist, designers, teachers and all creative pursuits. "
https://www.stockphotosforfree.com/ - "Here you can download over 100,000 completely free stock photos from around the world! We have photos that cover every topic from exotic locales to talent-released people and everything in between. There are never any fees to download our stock photos, and they come with a royalty free license so you can use them in all types of media, for worldwide distribution, forever! You can even use them in commercial projects-always free!"
http://www.freeimages.com/ - "Whether you just want to browse our huge gallery or want to share your own photos with other, this is the site for you! Browse through the categories of our huge gallery containing over 350,000 quality stock photos by more than 30,000 photographers! Need a wallpaper for your desktop or illustration for your blog? Need a pic for your commercial website design? Looking for inspiration? Have a look around."
http://www.photober.com/ - "Photober is FREE of charge photo downloading site for personal or commercial use. All images are free to commercial reuse, and reuse with modification. You can download photos and do nearly whatever you like..."
http://www.rgbstock.com/ - "Are you looking for fabulous free stock images? Then look no further! RGBStock has a huge number of high quality free stock photos and dazzling free graphics for illustrations, wallpapers and backgrounds. Need a free picture for your commercial website? Want an image for your blog? Looking for ideas? Check out RGBStock galleries packed with high quality stock photos and find what you are searching for!"