"Circle" image shape not a circle in the "Writing – About Me" widget

In the “Writing – About Me” widget, I’ve selected “Circle” for the image shape but the resulting image is stretched and looks more like a pill or oval.

Are there dimension requirements for the image being used in order for it to appear circular? The image I’m using is a 300×300 px square.

Is there a custom CSS fix for this?

Thanks in advance.