I don't know if this is reliable, but Qwant (the search engine) lists images that have been designated commercially free to use. It's Qwant.com, popular in Europe, they say. Google had (had?) something similar too. I still list the source. Also, any US government site and wikia are all royalty free, etc.
You can go to Google and click on images and search by keyword. You can filter your search results to photos you can "share, even commercially" by going to Settings, clicking on advanced search, and resetting the license. You can click on the photo to get the credit, license info, and a link to the original page. Another option is to go to Wikimedia Commons.
Flickr is another option, but you need to always check if it is available and make sure to credit the source. Another publisher I used to work work required that the user name of the photographer as well as Flickr should be included in credit.
I had written 35 hubs, featured 34 of them, but for the meantime, I have unpublished all of them. I have recently found out that using images from Google are not allowed, and sites like Getty and Corbis is suing a lot...