I know the answer to this (most likely). It's because ersatztv runs under http rather than https. If you load your Plex library in your browser via app.plex.tv the logos wont show up in the guide. If you load your library via 192.168.xxx.xxx:32400, however, they will show up.
A fix for this that should be easy to implement is the option to link to publicly hosted channel logos via e.g. imgur.com.
If you want to hack it, the logo references are in a file which you can find at ../ersatztv/templates/channel-guide/_channel.sbntxt -- I have not done this myself, but i've run into the same annoying problem, and winnowed down the most likely cause to this. I've debated tinkering with it, but haven't done so yet.