It's not possible to change the default layout of PRTG, but you can create your own dashboards, by using the Maps or Libraries feature.
In a map you can also edit the background by clicking on the desired map and open the "Settings" menu. In here you can change the "Map Layout" by using you desired image or change the background color.
Please use the links above for more detailed information about these features.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.