Check out these two statuses:
They are derived from the same tweet. Under entities-> media you see that the display_url is different for each, but the actual media_url is the same.
It is a retweeted pic, but you changed the pic url, making it very difficult to group them. You are using the pic.twitter.com to simply mask your t.co, but then you aren't reusing the url when someone else tweets the same pic. In short, you aren't doing it the same way as other pic sites, and it causes problems for a number of people.
Here is an example of your own search not working so well:
If you expand the first bunch of photos you will see they are all the same.