Marvelmania published a "self portrait kit" from Marvel's top artists. I saw the ads for these and always wanted to buy them. Trying to collect the image scans for them isn't easy, either! Thanks to Wendy Patinkin on Facebook, I found a few more.
John Romita with the Spider-Man cast. This is the most well known of all the self-portraits.
Recently, Tom Brevoort published the original art to this piece on his snazzy Tumblr blog, The Marvel Age of Comics.
John Romita's biography that was also published in this set.
John Buscema, with the Silver Surfer and Mephisto.
The above scan is just one part of a much larger piece. I have Buscema's sketch for that, but not the final illustration.
Gene Colan: Iron Man, Doctor Strange (masked superhero version), Daredevil, Captain America, and Sub-Mariner diving into a water glass!
Herb Trimpe, with a Rampaging Hulk, Nick Fury, Phantom Eagle, Ka-Zar, and others.
I am missing quite a few pieces here: the full John Buscema illustration, Jack Kirby, Jim Steranko, Marie Severin, etc. If you find any scans of these, please send them my way. Nuff Said!