I use the site http://www.picons.xyz for the source of the icons.
Install steps i took:
wget https://picons.xyz/downloads/binaries-snp-full/snp-full.800x450-760x410.light.on.transparent_2016-06-03--12-46-43.symlink.tar.xz
tar -xJf snp-full.800x450-760x410.light.on.transparent_2016-06-03--12-46-43.symlink.tar.xz
cd snp-full.800x450-760x410.light.on.transparent_2016-06-03--12-46-43
sudo mkdir /home/hts/picons
sudo mv * /home/hts/picons
sudo chown -R hts:hts /home/hts/picons/
Note: download link is date based, so check the site for the current link.
Now you can set the picon folder of tvheadend to file:///home/hts/picons
And you can set the filename of the icon to picon://rtl5.png (for example) for the channel.