Calling getimagesize() returns an array:
Array ( [0] => 300 [1] => 200 [2] => 2 [3] => width="300" height="200" [bits] => 8 [channels] => 3 [mime] => image/jpeg )
The value in index 3 is a text string that expresses the image's width and height.
So:
<img src="image.jpg" alt="Descriptive text" <?php $dimensions = getimagesize("image.jpg"); echo "$dimensions[3]"; ?> />