Tutorial Get Width/Height of Image
If all you have for an image is the URL, you can still find the dimensions:
<?php
list($width, $height, $type, $attr) = getimagesize("url/to/image.jpg");
echo "Image width " . $width;
echo "Image height " . $height;
echo "Image type " . $type;
echo "Attribute " . $attr;
?>