Preview images feature in the news and article grids, such as the spotlight or news section on home. If you don't set a preview image, you'll see an excerpt from the content.

Both news and articles use the same format, in the CMS when creating or editing news/articles, find the Preview Image input box and upload an image in either PNG or JPG format, with a max resolution of 1920×1080px. Generally JPGs are best for photo-style images, and PNGs for graphical images.

These appear in the course list, as well as larger in the course itself.

When creating or editing a course, you can replace the placeholder image with one of your own. This needs to be a JPG with a maximum resolution of 1280 × 720px.

A good free image resource that is royalty free is called Unsplash.