Find free, high-resolution images
There are many sites on the internet with collections of free, high-resolution images to use in your courses. A quick search yields quite a few resources:
Ruzuku doesn't endorse or recommend these sites; they're just some examples of what you can find with a simple online search.
Note: Be sure that the images you use are licensed for commercial use. It makes sense to read the full license description before downloading and using any images you find online!