How to display image height and width in pixels?
Have you created an image style for the image (what used to be ImageCache in Drupal 6 now in Core)?  Configuration -> Media -> Image Styles

If not perhaps you should because this will give you much more control over the size of an image without having to worry about theming the size directly.  Set up a preset for the image display in question and you can choose the size it should be generated at for a particular use.