Could you please clarify where do you need the image to be shown - as a background to the whole dock or to its content only?
The following example from our online demos shows how to hide dock's title and borders and set background image to the dock's content: http://demos.telerik.com/aspnet-ajax/dock/examples/highlightedcssclass/defaultcs.aspx
In case you want to keep the title and set a background image to the dock's content container, you should apply the following CSS:
.rdBottom .rdCenter, .rdRight, .rdLeft
Please note that the above CSS code will set background image to all docks present on the page. You should use ID selectors to set different background images to RadDocks. For your convenience I have attached a sample working project that uses the above CSS.
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