You could try using a custom content provider for Sharepoint. Please, check this demo for more information on using custom content providers in the RadImageEditor - http://demos.telerik.com/aspnet-ajax/imageeditor/examples/default/defaultcs.aspx
Hope this helps.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed