How can I find the width/height of an IMG/Image?

The pixel width/height of an image are readable in NN3+ and IE4+:
document.imageName.width
document.imageName.height
for <IMG NAME=\"imageName\" SRC=\"img1.gif\"> respectively
var img = new Image();
img.src = \'img1.gif\';
...
alert(img.width + \'x\' + img.height);

Note that an image has to be loaded to have the width and height
available so consider using the onload handler e.g.
function showDimensions () {
alert(this.width + \'x\' + this.height);
}
var img = new Image();
img.onload = showDimensions;
img.src = \'img1.gif\';
Note further that in IE4+ and NN6 you can not only read but also set
the width/height of an IMG element.

IE4+ also has a fileSize property for IMG element/Image objects giving the size in bytes. Thus
var img = document[\'imageName\'];
if (img.fileSize)
alert(img.fileSize)
will show the file size if the property exists.

In NN7 (6?) / Mozilla you can get the original width and height of an
image by:

img = document.getElementById(imageid);
alert(img.naturalWidth+\"x\"+img.naturalHeight);

these values may differ from img.width and img.height due to CSS
settings or altering the image size with javascript.