Depends on whether you're comfortable with using telnet/ftp or not. Two options:
1. There are picon sets available on the feeds (download plugins and navigate to picon sets and pick one), which you can specify to install on the flash. You just need an enigma restart to activate.
2. Alternatively you can telnet into the box and copy the picon folder. I have a backup copy of my picons stored in a folder on my HDD called /backup_picons/picons
What I do is telnet to the box and issue the following commands:
cd /hdd/backup_picons (this places me one directory level above where the picon directory is stored on the HDD)
cp -r picon/ / (this copies the /picon directory and all its subdirectories to the root (/) of my flash)
Once you restart enigma the picons are active.
Make sure you have plenty of flash ROM free on your box. Some picon directories can be very large if you have downloaded a picon set covering lots of satellites, so check sizes first.