How does "Site not allowed to load images" work?

[Matthew Cline]

I have ad.doubleclick.net in my list of sites that aren't allowed to load 
images.  I thought that this would keep Mozilla from even trying to load 
images from Doubleclick.  However, Mozilla still tries to connect to 
Doublleclick, since I I get a "connection refused" dialog when it attempts 
the connection (I also have doubleclick.net resolved to 127.0.0.1, so a 
connection can never happen).  So why does Mozilla try to connect, when 
it's not supposed to display images anyways?

Thanks in advance.