I’ve been seeing posts about this since 2015, and when did Plex add photo support? :-} Something, presumably the server, is recompressing photos with low resolution and quality settings. On any retina device (isn’t that all of them these days) the loss in sharpness is immediately noticeable on viewing, and if you pinch zoom to look at some detail it is a mess. Why on earth? Every 2-cent DLNA client out there does a better job. Too bad they don’t do all of the other stuff that Plex does, like thumbnailing, album styling, browse position remembering, and more.
Please Plex, just send the original images directly / at highest quality when viewing on LAN, and give user a choice when remote. Isn’t this how you do it for video?