Note: it is invalid to provide percentages directly as <img> width or height attribute unless you're using HTML 4.01 (see current spec, obsolete spec and this answer for more details). That being said, browsers will often tolerate such behaviour to support backwards-compatibility.
Those percentage widths in your 2nd example are actually applying to the container your <img> is in, and not the image's actual size. Say you have the following markup:
<div style="width: 1000px; height: 600px;">
<img src="#" width="50%" height="50%">
</div>
Your resulting image will be 500px wide and 300px tall.
jQuery Resize
If you're trying to reduce an image to 50% of its width, you can do it with a snippet of jQuery:
$( "img" ).each( function() {
var $img = $( this );
$img.width( $img.width() * .5 );
});
Just make sure you take off any height/width = 50% attributes first.