First off -- not Gandalf, even if the hat is kinda similar. It's just a wide brimmed, pointed, medieval hat -- in this case, worn by the Norse god Odin and it's brim pulled down, on one side, to hide his missing eye.
This picture is a little gruesome, but I don't think it quite dips into the "mature content" category, containing nothing you wouldn't find described in a thorough book of Norse myths. Sacrifices were made to Odin by hanging the victims to symbolize Odin hanging himself (but not necessarily by the neck) from the great World Tree Yggdrasil for nine nights to acquire knowledge of the runes (note the rune-carved stones). The heads at the bottom of the picture symbolize those killed in battle, whose spirits go to Valhalla and will eventually fight for Odin during the final battle of Ragnarok.
Odin is atop his eight legged horse, Sleipnir, and carries his spear, Gungnir. His ravens Hugin and Munin alight on his shoulders to tell him what they have seen on their flight.