How to limit a draggable image to a certain area?

[William de Smet]

Hi there,
It's been a while but I've got another question now:
In my app children can drag fields onto another field and see a reward.
But while testing someone told me that the image could be dragged out  
of the main window and wasn't visible anymore.
So my question here: how can I limit the draggable area so the image  
cannot be dragged out of the main window again?
TIA,
William de Smet

The code I use so far:
on mouseDown
   grab me
   set the layer of me to 100
   set cursor to 28
   lock cursor
   end mouseDown
on mouseup
   unlock cursor
   set cursor to arrow
   if intersect(me,field "Woord11") then
  move me to the loc of field "Woord11"
  play audioclip "pop.aiff"
  add 1 to field  "score2"
end if
send mouseup to button "check"
end mouseup