I had emailed Billy to ask about the skull tattoo (thinking it was on his arm). Here's his reply with interesting link - Andys' birthday....
"hi. the skull tattoo on the velvet underground album cover was from an actor in andy's 'bike boy' movie, joe spencer. i have a black panther tattoo on my arm since i was 15 years old. i'm 65 now; 50 year old tattoo! check out: http://www.warholstars.org and click on 'news' on the front page to see the latest. billy."