How can I draw a trasparent pixmap when drawable is a pixmap?

[rings <rings etang com>]

Hi All:
    I have a question about transparent pixmap.I have a big pixmap,now,I want to get this result.When I press 
my mouse button,I draw a small pixmap on the big pixmap.In other words,my drawable is a pixmap. But my 
difficult is that a small pixmap's size is 48x48 and it have white background color.I draw it on a big 
pixmap,then I will find its white background color hide big pixmap.How can I chang it transparent?  

thank you.

rings