Currently, MastoPlus appears to be one of the best social media websites to upload large/tall images; however, it is still rendering images at a smaller size then the photos intrinsic size.
Within the newsfeed, it has to be rendered at a tiny fraction of its intrinsic size, and that makes perfect sense. However, when clicking on the image, it can still be rendered at a significantly smaller fraction of the intrinsic size of the image (about half the size).
Currently, it seems the behavior of images that are too large for the screen size is that it will match either the height or width and scale the other side down proportionately. My suggestion, for when the image height or width is too large for the current viewport, is to display the image at its full intrinsic size and allow for scrolling (potentially allow zooming, too.)
While the images can be opened in a new tab or saved on mobile, which usually allows for complete intrinsic size rendering, this is occurring outside of the UI. My audience doesn't always know how to do these steps and the images are impossible to read for them. 1st-party support of intrinsic size rendering is preferred - that means one click on the image and it is rendered at its intrinsic size.