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 http://www.online-image-editor.com/ if you want it to be rounded border) and then from Custom Preview browse it From Local File.