Only images can be added to panels, but other elements can be combined with panels in interesting ways:
Clipping
Choose Arrange->Clip if you would like an element on top of a panel to be clipped to the edges of the panel. This is not enabled by default since many photos do not provide room for the text you'd like to write. However it can provide a crisper look:
Notice that the text is fitted to the combined outline of the panel and the balloon when clipping is enabled.
Snapping captions
When captions are dragged onto panels they will snap to the edges of the panel for a clean look.