It is a know issue with RadDesktopAlert
in the latest version. The issue has already been addressed and the fix will be introduced in the next official release. Here is the feedback item for your reference: https://feedback.telerik.com/Project/154/Feedback/Details/243462-fix-raddesktopalert-the-content-image-cannot-be-set
Currently, the possible solution that I can suggest is to set the RadDesktopAlert.Popup.Image image property to the desired icon.
I hope this information helps. If you have any additional questions, please let me know.