External jpg width

How can I find the width of an external jpg BEFORE it’s been loaded into my movie? I can easily find the width of it AFTER loading by loading it into an empty movie clip and returning the width of this AFTER the jpg has loaded using the _width command. But what I want is to find the width BEFORE loading.

I have tried “trace(1.jpg._width);” but this returns undefined, obviously as 1.jpg is not an object from my library, it’s an external file. Any other ideas anyone?!