For me it only changes if I touch the pic itself. If I scroll the post by other means, the ratio remains wrong. Was never an issue until the latest few versions of Jerboa.
That's how it shows up for me at least. Reduced aspect ratio and the top/bottoms of images are cropped to a standard size. I have been needing to wait a second for the image to fully load it's full format rather than needing to interact with a post though - it just happens automatically when it's in view on the screen.