Hello Jo Bert,
The MOSSImageManager is the default image picker dialog in MOSS 2007. I don't think you can configure it from out editor. On the other hand, it seems strange that you manage to upload the file using our Image Manager dialog. SharePoint 2007 works with impersonation, so the upload operation will be performed with the credentials of the currently logged in user. If they have contribute permissions for the list, the file will be uploaded. Otherwise nothing should happen.
Perhaps the access denied error is not because the user has no contribute rights for the list, but because they do not have read rights for one of the parent sites. I think the editor finds the list by starting from the site collection level and it checks permissions on each step. Please see if this is the problem in your case (i.e. user does not have rights for one of the parent sites).
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.