<img src="path/to/mypic.jpg" <?php $a = @getimagesize("path/to/mypic.jpg"); echo $a[3]; ?> alt="My pic" />

The lazy way to generate XHTML-valid width and height attributes for your images. This is incredibly inefficient, though I believe (ref?) that calls to getimagesize() are cached.

Automatic image width and height tags

images

PHP