Here are some sites that offer free usage, public domain, high-quality images. As well as an article from 99 designs giving their top sites for image searching.

+ Unsplash
+ Pexels
+ Pixabay

Article: 99 designs: 31 Free Public Domain Image Websites (Use with Care)

Google, of course, comes to mind first. With Google, remember that many of the images that appear in searches are subject to copyright and come in various qualities (some great and some not so great).
For more accurate search results, after Google searching for an image, go to Settings > Advanced Search. There you can specify the image size, aspect ratio, colors, file type, etc.

Happy searching!