I wanted to add to this. I paid for mine through a site called DepositPhotos. There are other sources one can get free pictures but I have lost the names/links. If you don't mind paying for them, appsumo will run a promotion once or twice a year for DepositPhotos where you get 100 for 39.00. You can download them as you need them. I bought up a few plans over the years for the books I was publishing which is why I am using them.
England has some sort of government library that offers copywrite free images, but I can't remember the name of it.
Thanks for your contribution! @practicalthought
I will do well to check out the DepositPhotos site you mentioned, probably use a search engine to locate the actual websites as well as look for other sites with a similar function.
Do have a great day! :D