Long listen but a very thorough and nuanced discussion by a bunch of smart investment / finance guys in Canada. No click-bait-sky-is-falling content.

Starting at the cheaper/low end you have 'free' imagery (some flavor of Creative Commons Zero [cc0] license) that lots of sites listed here offer. The one caution with free (as I think someone else mentioned below) is that the licenses aren't always clear (what you can and can't use it for) and in some cases the attribution / provenance of the image is wrong / unclear. Meaning that what you think you are ok to use, may have actually been appropriated from someone else; and you are potentially infringing on copyright. I haven't spent enough time on the different free sites to see what their policies are regarding provenance, so your mileage may vary.

Then you have the microstock stock imagery - images that are in the $1-$5/range. Companies in that realm include folks like Shutterstock, 123RF, Dreamstime, and a slew of others. Shutterstock's public filings says that their per image license is something like $2.25 (or it was when I looked maybe a year or two ago). These images are often the ones that people refer to as 'stock' in that they look like stock. Not all of course, but I am sure you know what I am talking about.

Then you have the realm of 'midstock', which is somewhere between (you guessed it) the low end and the highend. There are a lot of players in here, but iStock (from Getty) is probably one of the biggest ones; as well as Adobe's offering (from when they bought a library called Fotolia). This area of the industry is increasingly being carved out as prices either go down down down, or the more unique and premium imagery hold their own.

At the high end of the stock world you have what is called the 'premium' stock photography folks - this includes Offset (from Shutterstock), Getty Images, and the new Premium offering from Adobe. Those are the three big players at the top end of the spectrum, and then you have a lot of smaller studios that sell directly to end consumers and also place their imagery with the big three. So in some cases you can find the same imagery across a lot of different providers.

My background is a photographer and I have images with a bunch of these more premium folks and looking at my royalty statements the average sale is more in the $110 range/image. Or ~50x the low-end of the market. So it really depends on how much $$ you are looking to spend and whether the quality of the image (beauty in the eye of the beholder and all that) is important to you.

The other site (full disclosure, I am one of the founders) to check out is https://haystack.im that aggregates from a couple dozen different stock agencies all in one place (including several listed here like Stocksy, EyeEm, 500px, Cavan, Maskot, ImageSource, and a science-focused site called SciencePhotoLibrary). You can pick one/several agencies to search at any given time and then we boot you off to them for the final license. So we are more like a premium stock photo search engine than a distributor of the stock itself. Think Kayak not Delta.

Hopefully that makes sense. Hit me if you have any stock industry/photography questions. I am at: andrew@haystack.im