Having researched this recently in my line of work, I happen to have a list of sites handy. It seems new users can only post 2 links per post, so you’ll have to add “.com” to each of these:
unsplash realisticshots pixabay lifeofpix gratisography freenaturestock snapwiresnaps.tumblr publicdomainarchive travelcoffeebook furiouscamera jaymantri kaboompics startupstockphotos freenaturestock fancycrave pexels thepicpac barnimages crowthestone stock-image-point photocrops resplashed skitterphoto jeshoots
Those all used CC0 (Creative Commons Zero), which is the most permissive license I’m aware of.
Getty bought an Israeli startup called PicScout a few years ago and they use their web spider software to find images they own and send settlement demands to the tune of say $800. And there are Getty-owned images suspiciously planted here and there among the free sites. I recommend doing a search-by-image on Google to see if the image is listed on any Getty-owned sites. (iStockPhoto, BigStock, etc.) If so I would avoid it unless you paid them for it directly.