The Image number (and also serial number of the camera!) tags are only visible if it is a NEF (as said it's own expandable list under http://ns.adobe.com/exif/1.0/aux. It is NOT a a sub category under the EXIF Properties)
However if you view the EXIF of the JPGs with Opanda, you can view an actuation number. The Entry is called Total Number of Shutter Release for Camera. It is near the bottom of the list.