Hello Lorenz Buchberger,
Dropping in a panel should be possible. Sometimes panels do not receive mouse events because they do not have a background property set, try to have a recognizable background while debugging and at least "Transparent" otherwise.
Also please make sure that the panel is stretched, this should be visible once you set a background.
As for your second question: The e.Options.DragCue of the DragDrop can be anything, including an Image control whose Source property is set to a RenderTargetBitmap. Then you can use the Render() method on the RenderTargetBitmap to render an arbitrary control there.
You may see other solutions that are using a VisualBrush but in WPF anything that you assign as its source will never be garbage collected (i.e. it will result in a memory leak).
the Telerik team