When this option is set to the URL to some image, it will be displayed in the top-left corner of the Virtualmin user interface, when using the framed theme. You can set it to the logo for your hosting company, which when clicked on will open the URL specified by the Link destination for company logo option.

Although the exact allowed size of the image depends on the browser being used to access Virtualmin, the image should be at most 220 pixels wide. The height is not limited, but 120 pixels seems like a reasonable size.