If icons cannot be read, just write the name of the channel with letters, which is what is happening if tuner does not provide icons. That way at least we can see which channel is which.
You know that tvh is something which is not officially supported by Plex? You might try to reach out to other users in thread of the plug-in you use for accessing your tvh host
Edit: Second thought, the issue might lie within your xmltv file and how Plex treats it. While I was fiddling with xmltv guide data, I also had the impression that this is not something which is well supported within Plex. It works, but issues are often not addressed.
Do you feed the xmltv file manually into Plex or is this somehow provided by tvh plugin?