C# Irregular shaped user controls.

How to make usercontrol have transparency key for its background image? I
want to have irregular shaped user controls on my form. like a funny shaped
button. This works fine for irregular shaped forms becuase they have a
transparency key field, but user controls dont have this field. note i do
not want to see the desktop. I want to see any other controls or form behind
the irregularly shaped user control. And i want it to remain looking ok when
i resize the irregularly shaped user control.