These broken images could be due to a problem with the global CSS class.
I believe that Telerik made some changes recently in the skin rendering functionality in order to allow rendition of multiple editors on the same page with different skins.
Try to test the RadEditor using a new application that uses the full tool set. You can do this by porting the Editor/Default Settings
online demo to your own system. If it is rendered correctly with all toolbar icons showing, then I would suggest that you remove the stylesheet from the problematic existing application in order to see if this will fix the issue. If it does, then you will need to isolate the problematic global css class and fix it.
If you're not able to fix it, I'd suggest that you open a support ticket directly with Telerik. And, if you do so, maybe you can provide a live url or a project that they can test with in order to more easily identify the reason for your issue.
And, another option to fix this would be to install Fiddler
in order to inspect your page to see if there are any blocked resources. See this forum thread for insights: Icons missing in RadEditor toolbars
Hope this helps!