Low resolution photos

Hi, Since a couple of weeks, my uploaded photos appear in really low res on all my devices using Plex. So also on my Samsung TV (49KS7000 UHD). the resolution is appr. 480p. A month ago, it was really 4K what I saw. I ve tried everything, checked my router (airport extreme), installed Plex again, set the transcoder settings high, etc. uploaded my photos again, nothing helps. When I look in the information section when showing the photo on TV, it says that the photo has 0kb.

An uploaded 4k video plays fine by the way, even more strange???

Does anyone have a solution for this?
KR. R. Verhulst