Rectangular & Circular Image Mask
This widget use the CSS inset box shadow property to create an image mask.
The first example has a small corner radius and a contrasting shadow color. A screenshot shows the widget settings and the caption is dropped on using the Google Headings widget.
The second example has a shaow color the same as the page background and the corner radius is set to create a circle.
To create a circle from a square set the corner radius to 50% of the width or height. In this case the image is set to 400px square so the corner radius is set to 200px.