Actually, that "Satanic" hand gesture was created by none other than Ronnie James Dio, from the band "Dio" (and Black Sabbath, and Rainbow). That link provides the album cover of his first solo album. Definitely one of my favorites.
Satanists believe in neither God, nor Satan. They use “Satan” as a metaphor rather than a deity, and focus on the spiritual advancement of “The Self” rather than submission to a set of moral codes. The “Church of Satan” founded by Anton Lavey is a recognized religious institution, even eligible for tax exemption