I've never had success using "picons" in TVHeadEnd. If you use Kodi you have the option to store them locally in a directory on the same machine that runs Kodi, and that's what I do. In Kodi go to System | Settings | TV | Menu/OSD and then click on Folder with channel icons, then you can set the path to wherever you store your channel icons. The advantage is you can use icons with better resolution than the typical picon (you can often find icons on Wikipedia or using a Google image search). If you are not using Kodi then there may be a way to do something similar in whatever software you are using.
Note that to remove a picon link in TVHeadEnd you have to set it to a single space character, then save it. If you are going to use icons on your local system you probably should remove the picon links.
Also note this may vary depending on what version of TVHeadEnd you are running, I have a slightly older version so things may have changed somewhat