I believe the reason for using a fixed height was that it prevents the layout from jumping when loading new images. There should a better solution I guess. Somehow twitter manages to have sane image proportions for gifs/videos w/o the layout jumping on feed load while not resorting to the fixed height usage.
Maybe delaying the rendering of the feed until the media attachments are loaded would be better UX-wise (shouldn't be too long w/ lightweight thumbnails/previews), will try that
After trying different approaches, I think defining padding-bottom in percentage is the best one. It allows to specify the height of the image placeholder relative to the width of the feed and we can easily infer it from the dimensions provided by BE. This way the original aspect ratio of the image is preserved, the image height is tied to the width of the feed and is updated when it changes, and we don't need to wait for the image to be loaded to display it (thus, no delay and no layout reflow / jumping when the images are loading). (MR !75 (merged))