ImageSizeInfoFunctions is an extension to MediaWiki that gives two parser functions returning width and height size of a given image, available in the wiki.
To install this extension, download the extension into a folder called ImageSizeInfoFunctions under the extensions directory then add the follwoing lines to the end of the LocalSettings.php file:
// ImageSizeInfoFunctions
require("$IP/extensions/ImageSizeInfoFunctions/ImageSizeInfoFunctions.php");