Facebook "FBCDN image is not allowed in stream"?

after uploading an image, i want to post it on the users wall. Unfortunately, i get the following error within a Facebook UI:

"FBCDN image is not allowed in stream.filepath"

It seems that the value @"picture" won´t be accepted, but why? When i NSLog the "thumbURL", its correct. How can i fix that? Here´s my complete method:

    NSString *thumbURL = kDefaultThumbURL;
    NSString *imageLink = [NSString stringWithFormat:[result objectForKey:@"link"]];    

    NSMutableDictionary* dialogParams = [NSMutableDictionary dictionaryWithObjectsAndKeys:
                             kAppId, @"app_id",
                             imageLink, @"link",
                             thumbURL, @"picture",
                             @"imageName", @"name",
                             nil];

    [appDelegate.facebook dialog:@"feed" 
                       andParams:dialogParams 
                     andDelegate:self];

Thanks for any help..