I’ve already mentioned that above:
The web browser prevents loading of pictures from http, if the web app has been loaded from https. That is a policy of your web browser, not of Plex.
The local Plex web app (as opposed to the hosted web app) is only loaded per http, because there is no valid security certificate for your NAS. So it might work in this constellation.
If that were the case. you wouldn’t see logos under any circumstances. Those logo picture links cannot come from anywhere else than the EPG, which in your case is the XMLTV file. So look at it in source code mode.
