Per inspiration provided by Brian Harvey.
It's cool! But is there a way to delete the part of the picture that is outside of the picture crop lines? Or do you have to look into the coding to do that?
I think the user interface needs work. :~) The initial position of the crop lines at the edges of the stage made it hard for me to drag them. I was imagining using arrow keys for positioning, finding a way to move by tens until clicking somewhere makes it be by ones.
I came up with this option to replace the goto 0 0 block so that the cropped image stays inside the crop area as I think it looks bit nicer.
Chopper Sprite is ony the cropped part.
You can thicken the lines for easier aim.
I'll add the arrow movements in the next RELEASE .
Now displays the cropped area's size.
And incorporates @cymplecy's suggestion of displaying the cropped image within the cropped region.