Getting my head around resizing images

I have a main image on the first screen of my flash website. The size of the flash site is set to 800 x 600.

On larger moniters the flash site looks fine (obviously cos its vector based) but the main image degrades pretty rapidly if the site gets stretched.

I know how to upload an image using loadMovie and keep it separate from the flash movie file but what I can’t get my head around is how the resolution will change depending on the size of the flash site within different browser windows.

If I set a movieclip as a place holder for the image and then get it to load an external jpeg file that resizes to the place holder dimensions what happens if you stretch the size of the place holder. If a flash movie is resized according to a browser window, do the values of the dimensions of the flash symbols change or are they always set at the size they were created?

Whats the best way to get an image to resize with the flash website and keep the resolution at a maximum at all times?