It is possible to have unique configuration files (one for lists and one for web parts and publishing fields) for each SharePoint web on your server and in this unique configuration file you can set certain properties for RadEditor - ImagePaths, FlashPaths, DocumentsPaths, MediaPaths etc.
For more information please, review the following help articles:
Note that your ImagePath can look like this: SitesCollection\Site1\SiteCollectionImages
I hope this helps.
the Telerik team