These site don't provide royalty free images, whatever that means. These images are free,as in no money is transferred from you to them, as in you can download and use them as you please. Sometimes you have to give credit to the photographer, but other than that they are gratis. We've used them all for our Amazon FBA Shopify store and our favorite has got to be Unsplash.
We like Unsplash because the pictures are gorgeous. The images trend towards landscape-ish shots, nothing too specific, but exceptionally well done.
Whimsical and fun, these images run the gamut from goats to urban landscapes. If you need something unique and different go here.
Life of Pix is great, just a little more mundane. If you need a desk shot, or someone sitting on a park bench this is the site for you. Excellent well executed imagery, and no spam. Great site to visit to see what they have.
There are obviously more sites out there providing free high resolution photography. What we've found though is that most of the others simply re-hash these sites and can contain spam and ad-bots, so we avoid them like the plague.