Additional styling to images using the Media Library can also be set.
Navigate to the Style tab of the Image subsection
Scroll to Image
Image Width and Height - If you select this checkbox it will automatically make the width and height of the image the same
Border Radius - Adding a radius will make the image corners round
Use both options to make an image completely circular
To Note
These settings are for images that are added as content, they will not be applied to background images