You’ll often see a circular image used for profile avatars, like the one at the bottom of this page. In the past, this was a bit of a pain to get rendering correctly across all browsers and devices. Nowadays, this can be easily accomplished with the border-radius CSS property.
For good measure, let’s ensure the image is a square - the width and height should be equal. If your image isn’t a square, you can crop the image so that it becomes a square. If you’re not sure which program to use in order to crop your image, I would recommend trying GIMP.
Suppose we have the following image, 200px width by 200px height:
With the following, we can make the square image into a circular image!