John had six tattoos done on his arms in the period from 1988 to late 1990 (nowhere else, unlike the rest of RHCP who had their backs, legs, chest and even heads tattooed). Most were damaged after his '90s drug abuse and one of them has completely disappeared. These images should give you a clue about where and what they are/were.
This is the only tattoo John has that's still in its original shape, it was not damaged. It was designed by Henk Schiffmacher also known as Henky Penky and it's a take on Native American flashes.
Left Inner Wrist
The face was inspired by the cover image on free jazz musician Ornette Coleman's album Dancing In Your Head. You can view the original image here. Right above this tattoo, closer to the hand, there's another - a small face, in a style similar to the left shoulder design.
This beautiful octopus was designed by Henk Schiffmacher also known as Henky Penky and it's meant to serve as a souvenir from the Positive Mental Octopus tour the band did to promote their Mother's Milk album. Chad Smith has had a more stylish octopus done, but on his thigh.
Right Inner Wrist
This is the same design that Anthony Kiedis has in the same place - just RHCP's symbol, an asterisk. Many Red Hot Chili Peppers' fans around the world had the very same tattoo done on their own wrists or elsewhere on their bodies.
Right Outer Wrist
This design represented simplified symbols of a man and a woman, having sex. It's gone now.
* Owner of invisible-movement.net hates this page (which was not updated for a decade), but decided to keep it public because people ask about John's tattoos a lot.