What has always bothered me about MediaWiki is that large images are not nicely integrated; instead there is a small version, along with image information (which is fine), and then there is a link to the full resolution image. The problem with the full resolution image is, that it is unhandy in most browsers to view the image properly.
Yes, there is the preview, but I would love to see the full image and navigate within it, in order to see the details.
This may sound like a rather minor issue, but especially for a media library such as the WM Commons, I have always wished for a proper image viewer. And -- good news -- so I wrote one! ;)
XXLview (http://www.zirona.com/software/xxlview/) is an application I wrote especially with the WikiMedia Commons and large images in mind. So it might be worth a try to integrate it into MW. Integration is fairly easy; instead of linking directly to the large image version, the XXLview template is loaded with the image inserted. (Due to the MIT license, the template can be fully rebranded for MW.)
If desired, I could develop a patch against the latest MW version and post it here.