Browser icon

The Site settings > Site content > Browser icon is a small image that will display in the visitor's browser tab, and sometimes site bookmarks.  It can appear like so:

Image file type
Upload a jpg, gif or png file that is greater than around 200 x 200 pixels.  This image will create a multi-purpose site icon.  For example, if a visitor turns your website into an icon on their mobile device, this image will be used here as well.