Constraint proportion problem

Greetings,

Im developing a full browser flash website for a photographer and I should make the pictures really big. Now my problem is that when you see the website in a smaller resolution than the one im using, images get too big. What i need is a function that resizes the image box according to the stage size, but I dont know how to constraint the image proportions.
For instance I would make something that would do imageMc._width=Stage.width*0.8 so I could have the image with 80% of the screen size… but how to make it proportional with height? You get the problem?

Ok, for you to help me out on this is better to check the development:
http://www.stevestoer.com/teste/

Any ideas on the function?

Please help me out on this. Thanks…