I checked with the following code:
var a=new Image(); a.src='http://example.com/image.png';
alert( a.width );
which in this case returns 0 since the image doesn't exist.
On link text I found quite a few properties that might be worth checking out. And if that fails you might want to look into jQuery and AJAX.
Edit:
tested on firefox firebug
var a=new Image();
a.src='http://l.yimg.com/g/images/photo_unavailable_m.gif';
console.log(a.width); // the image exists and returns 240
var b=new Image();
b.src='http://farm3.static.flickr.com/2560/4098180849_729ef4f6ef_m.jpg';
console.log(b.width); // the image does not exist (re-direct) and returns 0
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