I mean the second one, going to the properties of the movie in Mezzmo. Since Mezzmo is showing the poster this means the problem could be in the transcoding folder, Mezzmo transcodes posters for devices to adjust the size. Please go to 'Media Devices' in Mezzmo and double-click on the device that corresponds to the computer running Kodi then on the Transcoding tab click the button to delete transcoded files then go to Kodi and see if the posters appear. You can also try installing the Mezzmo Kodi addon in Kodi if you are not already using this to see if this shows the posters as it is possible that Kodi has cached the information and is using old URLs to try and get the posters.