Gandalf's Laughter

Even though he is depicted in white, the surrounding ground and spirit of the laughter would lead me to believe this is when Gandalf finally remembers the password for the gate to Moria. Although, it could also be a few other times later in the story, perhaps after he is resurrected and meets up with Aragorn, Gimli, and Legolas on the fringes of Fangorn Forest. One other distinct possibility is when Gandalf laughs outright at "The Mouth of Sauron" in front of the Morannon (the Gates of Mordor). The only problem with this last suggestion is it would appear Gandalf is sitting down.

Book: The Lord of the Rings