Panning limits HELP!

ok … I have some problems figuring how to fix limits to a panning image.
I have this simple piece of code on a movie clip with my image inside of it :

_x += (320 - _root._xmouse)/40;
_y += (240 - _root._ymouse)/40;

with this, my image is panning around to stage relative to the movement of the mouse… but with this, the image continue to pan eternaly … I would like it to pan only in a “restricted” aera … this area would be the movie clip… so can anyone give me a clue on how to do it !!!

There is my .fla to give a better example:

thank in advance ^^