There is a really great stock photo agency that is not the usual stock image site. unsplash.com.
A hash, in general is, according to Wikipedia, a “function that can be used to map data of arbitrary size to data of fixed size”, e.g. turning an image into a bitstring that represents that image.
Yes, unslpash.com is a great site. I recommend that people don’t use stock images, even if they buy them, because the Google spider is good at seeing pictures (DON’T ask me how, it boggles my mind to think that) but there is a little trick to it. If you get a stock image, Google will say I have seen this in 200 other places on the web, Ho hum.
BUT, if you make a screenshot of this image, it changes the hash of that image and makes it new to the spider! Tip for the day. Ha!
There is also a perceptual hashing algorithm. A perceptual hash is like a regular hash in that it is a smaller, compare-able fingerprint of a much larger piece of data.
The perceptual hash image is much harder to spot and Google is probably much less interested in this level of duplicate images. In fact, there is evidence that Google is reducing their ban on duplicate content in other areas. For example, two different but very similar post pages from the same blog or website may show on the same Google SERPs page.
If you want some great images for your website, I recommend that you go to unsplash.com. I don’t think they have an affiliate program, because most images are free.