BitBlt question

I attached a picture which explains what I'm trying to accomplish. The pink part is part of the image loaded using bitblt API, the blue part is the background of the form. What I want to do is make the pink area invisible, so the form would show thru. I remember there was something with mask related to bitblt, but I don't really understand where to go from here and which constant to use to mask the pink area as invisible. Every pink pixel should be invisible. Thanks.

I don't know if this will help, but since no one else has responsed, I'll give you this link.

BitBlt Tutorial