Normally you need to download Enigma2 style picon set. Google shows many hits but hard to find suitable and up to date set. Neither I could find. :(
On the other hand Tvheadend generated picon file name is not always good. For example it is totally bad for satellites with west position. Furthermore other parts of the picon name may not match (frequency, namespace). If the generated picon name does not exactly match with the filename then no picon will be shown for that channel.
I use my Tvheadedit tool to assign picon names since it scans the picon folder and tries to find the appropriate file to exclude the above mentioned errors. If you do not have the right picon locally you can still use an URL to channel icon on the web. (It is also supported in my tool)