Here's an option that you can do:

1- Disable dials background in  Setting->Global Options->Appearance

2- Edit dial -> Custom Preview -> Browse users gallery-> Select a round image that you like (Make sure you already type the URL on top)

*If you can't find the image/logo of the website in Users Gallery you can just search for the logo/icon of your desired website in Google Image and save it to your desktop (edit it with if you want it to be rounded border) and then from Custom Preview browse it From Local File.