They have to be in a folder named picon (all in lower case). I just copied them all (all the xxxx.png files) from where E2Piconizer put them to root/media/usb/picon (on my usb stick plugged into the back of my box)
If you already have picons from another source you may already have a folder named picon on your hard disk and you may need to clear it of the previous picons first or just rename the folder and create a new picon folder.
Restart the receiver after you install the picons. This is because the picon folder is selected during startup.