Hi Antonio,
Thank you for writing.
You can add a background image to RadForm using its BackgroundImage property. Probably, you would like to set an image to the titlebar part as well in order to have the complete look and feel of the real aqua skin. You can find how to do this in the attached small example project.
Write me back if you have any additional questions.
Sincerely yours,
Martin Vasilev
the Telerik team