1.To start, simply go to images.google.com, type in a keyword that's related to the topic of your website, and hit "Enter" on your keyboard:
2. On the next page that appears, click on "Search Tools"
3. After you click on "Search Tools", you'll see more options appear. Click the option that says "Usage Rights" and then click again on the option that says, "Labeled for reuse with modification"
4. After you click on "Labeled for reuse with modification", the results will update with images that you're allowed to use and modify:
Instead of uploading your images to your site with random, non-sensical file names, try changing the file name to be something more keyword focused. The file name of your image is directly pulled by Google and other search engines to show your images in search thus increasing your SEO.
That's it...! It would be good to here where others source free images to put in their website content...