how to get the width and height of an external image

Hello,

lets say I want to get the width and/or height of this external image (it could also be an external file on my hdd): http://www.icatcher.com/wp-content/uploads/2013/01/Adobe.png

I create a button and add a onClick function that says

var externalImageWidth = ???
alert(externalImageWidth);

How do I do it (so to say dynamically?)  without adding the image itself to my library or put it on the stage?

All my attempts said "undefined" or "0" or they just didnt work.

Thanks in advance!!