pixmap selection with mouse

For the Canvas it's simple.
Let's say that your pixmap it's at the position x , y

If x - MouseX() < PixmapWidth(yourPixmap) And y - MouseY() < PixmapHeight(yourPixmap)
drawtext("in canvas")
end if

For the other stuff have a look in the wiki.
http://www.blitzwiki.org/index.php/ReadPixel

Tell me if it works I never done something like this.
Good Luck