Sorry I didn't mention this in my previous post, I didn't realize you were talking about the RadEditor outside Sitefinity.
From the Insert a document
dialog, you are able to upload in documents and custom libraries. From the Insert an image
dialog, you are able to upload in image libraries only.
Currently, there is no way to restrict users from uploading in a certain library, as permissions per library are not implemented yet.
In the official release of v3.2 you can use the Images & Documents module permissions and set the Create
permission to Deny
for a given user role. Users in this role will not be able to upload files at the Images & Documents tab. However, they'll be able to do so from the 2 editor dialogs. This has already been fixed for the upcoming SP1.
All the best,
the Telerik team