perhaps something here would help?
http://www.blitzbasic.com/codearcs/codearcs.php?code=2127
I wrote that so you need a separate alpha image, but you may be able to modify some things to get what you need. If loadimage does not work, then perhaps there is a way to use loadtexture to grab the alpha data. From there it should be a really fast change to the code. I will try to look into it, but I can't get to it for a bit.
edit: look here too for some more info.
http://www.blitzbasic.com/Community/posts.php?topic=63776#711890
http://www.blitzbasic.com/codearcs/codearcs.php?code=2127
I wrote that so you need a separate alpha image, but you may be able to modify some things to get what you need. If loadimage does not work, then perhaps there is a way to use loadtexture to grab the alpha data. From there it should be a really fast change to the code. I will try to look into it, but I can't get to it for a bit.
edit: look here too for some more info.
http://www.blitzbasic.com/Community/posts.php?topic=63776#711890