How do I should to draw a transparent pixmap on other a pixmap?

[rings <rings etang com>]

 Hi,everyboday:
   I ask a question about background color.Now,I load a pixmap on a drawing_area.I hope when I press mouse's 
left button,I can draw a hand to show current place.I implement these, But  my pixmap that I draw have a 
background color.I want to have it transparency.Question is that my canvas is a pixmap and it include many 
colors.I might load many different pixmap to as a canvas.How do I should to draw a transparent pixmap on 
other a pixmap?


Best regardes


Rings