Image proportion not correct when browser size is decreased

On a desktop a Twenty Twelve page where images are displaying one per post display fine. When I drag the browser window in so the width decreases, the photo in the first post at top does not display correctly. The proportions are wrong. The photos in previous posts below display correctly while dragging. Same when viewing on iPhone. A landscape photo will display as a portrait due to proportions not working. Photos below that display correctly.