It sounds like what you are trying to do may be accomplished better by using the same techniques as for drag-n-drop. Your current approach will only trigger a single mouse-down event so what you will need to do is get the "point" (x,y) position during the mouse down, track the mouse move event and get the "point" (x,y) position at the time of the mouse-up event. Of course this requires you to determine which picture boxes fall within the selection buy comparing their locations.