Maybe we won’t actually be needing this. After having given it a second thought, I think I know what the reason is. Please try to upload a picture that is not 900px wide, but rather 752px wide, and see if you will still experience the shrinking.
Ok, please create a new page (not a post), but do not add it to the menu or anywhere else. So that only you and I know the link. Then add the image to that page. And send the link to me via email to pavel at mauer.co