Thank you for your answers.
I'm not sure what SmartShare is but I'm using the Photo & Video app that was preinstalled and all the other photos are working even those also saved with the same version of Zoner but some scanned photos that were cut and saved are showing a broken picture. The all have the same DPI but different resolution since they were hand cut. Thumbnails are showing only when played from NAS, not directly from external HDD. Here are properties of some photos. The working ones are scans of positive film also edited and saved in Zoner. I'm not sure about the version as I don't have it on me. For sure it's not the latest version.
I tried opening one picture in Corel and saving it and it worked, but it cut the size by around 75% so loss of quality and I have hundreds of these photos. Is there another app on LG content store I can try if it's as you said old version from 2018?
EDIT:
I tried checking the metadata of different jpg files everything is the same apart from resolution and DPI but this:
All the working ones have Encoding Process: Baseline DCT and all the not working one have Encoding Process: Progressive DCT
Can this be the reason?