This feature is only available in the desktop editor.
There are two ways to add borders around images in an image content block. The first is to set the border for the image canvas. This will place a border around the entire image canvas, which is a combination of images and annotations.
The second way is to add a border around an individual image within the image canvas. This can be useful if you have multiple images on the image canvas and want them to stand out from one another.
Select the image content block in the outline and change the Image Border in the content object property inspector.