A post without an image will garner little attention or votes. So we must use them. It is dangerously easy to google a topic and find an image easily.
These are free image sites:
https://unsplash.com/
https://pixabay.com/
https://stocksnap.io/
https://www.pexels.com/
https://www.freeimages.com/
https://pixabay.com/
There are pay sites:
https://www.istockphoto.com/
These sites set things up so you cannot simply use the URL of the image, you must upload the image to the image server (by dragging it to the post area), instead. The use of the image URL as is is referred to as "hot linking."