Photos - strange behaviour and quality

Hi, I have photos on my server that I want to view on my Samsung TV. I have Plex app installed on the TV, so I can directly access the server. I can also use Plex for DLNA sharing of the content and view it in the TV via its DLNA client. Anyway, when I try to display my photos, the quality is terrible. I have the photos resized to the resolution of the TV, however, when I try to open them via DLNA, Plex seems to be showing some tiny thumbnail stretched to the TV screen. The strange thing is that this does not appear every time. Sometimes, some of the pics are fine but when I open them again, the stretched thumbnail is shown. This is when using the TVs DLNA client. When opening the pict via Plex app, they just look awful (maybe some resizing or compression is going on). I am not sure but maybe I also get the thumbnail. I can’t remember now as I rather used the DLNA TV client.
Any help will be appreciated. Thanks

Same issue here with my Samsung Q7 TV. Any ideas?