transparent pictures toward each other with correct clicking area

This is not about transparency. Whole control can be done non-rectangular.
Use control's property System.Drawing.Region Region.

Region is a drawing object which can be constructed out of GraphicPath.

myControl.Region = new Region(myNonRectangularPath);
//from this moment, your control is re-shaped
//of cource, the region should be within Bounds,
//if not, you will get intersection of those

Of course, control's clickable area will be its region.

Also, regions can be combined using set theory operations: Union, Intersect, etc. to produce shapes of arbitrary complexity (all from native GDI).

This way people make any controls non-rectangular including forms/windows (look at those audio players).

WPF provides similar features.