I'm currently running iOS 11.2.6 on an iPhone X and am having trouble storing .png files via iCloud Photo Sharing. If I have a .png image in my camera roll with transparency and choose to upload it to a shared album on iCloud, it will appear and display correctly under the "Shared" tab. However after a minute or so if I return to the same tab and look at the same image, I find that it has been automatically converted to a .jpg file with all transparency lost and image quality degraded. This also happens on Macs and iPads. Is there a way to prevent this? I've looked under the iCloud Photo options and can't find anything. (This is separate from the iCloud Photo Library option, which does allow users to change format settings).