RadEditor does not provide the described functionality out-of-the-box, but still there are two possible ways to achieve the desired behavior depending on the moment that the filtering will be applied:
When switching between Design/Html mode or submitting the form
You can create a custom content filrer where to replace the actual (admin) domain name with the target one. A similar approach is demonstrated in the following KB article for link elements, so you can use it as a base for your custom filter implementation: How to produce relative path for links when MakeUrlsAbsolute content filter enabled
While inserting the image from the ImageManager
You can handle the Editor's onClientPasteHtml event and modify the image's source attribute just before the image it is inserted. More details are available here: http://www.telerik.com/help/aspnet-ajax/editor-onclientpastehtml.html
I hope this helps.