Yes very easy, if you have Linux, download the resolution you want, the largest transparent ones work best for most skins. I prefer using symlink ones because you can link Picons and channel names.
Then SSH and
Tar xv picon.tar.gz or whatever you called it when you downloaded it and transferred it to the tvh host machine
Then point tvheadend to the folder you've extracted to and that's it.
May need to reset icons if you want to see changes immediately otherwise 1/2 hours it'll all be populated