The right image can make the difference between no engagement and lead-generating engagement with your ads, website, blog posts, social media posts, reports, videos, etc.
When it's not feasible to take or create an original image/photo, stock images/photos can come to the rescue, but finding the right image can send you on a wild goose chase.
You'll likely find that a couple of the free image sites below, along with one of the paid sites, meet most of your image needs. You can then stick with those in order to streamline image research.
If you're looking for infographics, memes, animated GIFs, etc., then check out the Graphic Design post in the Digital Toolbox.
Be sure to read this article about optimizing images for search engines, especially about using the right ALT text for your images.
Always make sure that your chosen image includes a commercial use license!