Jack's cover for the 1975 Comic Art Convention Program book. The image was reproduced directly from Jack's original pencils. Below that the Kirby/Sinnott cover to Fantastic Four # 50 (May 1966).
ROBERT STEIBEL
An unpublished panel of Thor's alter-ego Dr. Don Blake. Stan Lee wrote a message on the side "Sol. Save for inking sample." Luckily this was not used as an inking "test." Jack's Margin notes say: "Blake describes exhilaration of becoming Thor -- The surge of great strength -- and his thought of moving boulder."
Apparently Jack wanted the panel to introduce this flashback sequence in Thor # 158 (Nov. 1968), but Lee decided against it. Pages 7 and 8 below were first published in Journey Into Mystery # 83 (Aug 1962).
A page of original art from In Love # 3 (1955). You can see quite a few paste-ups and white-out on the page, so this appears to be a story that was "cannibalized" or pieced together from old, unused art probably in Jack and Joe Simon's files. According to the Jack Kirby Checklist (2008), this story was assembled from unused Inky comic strips, with additional material added.
Here's a scan from the original artwork of page 1 from Fantastic Four # 90 (Sep 1967). A strange piece, all of the characters (except the Torch) are looking downwards, and there is no action -- you'd think Jack would use this entire page as an excuse to do something explosive, instead of having the characters all sitting around shoegazing. 

You can see text in this thought balloon, but it's hard to tell if this is Jack's handwriting.
In this bubble the word "The" and "Rocket" looks like Jack's handwriting to me, so it does appear in the 1950s Jack was adding text to the original artwork. This book was published by Harvey so Jack may have been writing and editing the material, but I don't know if he worked using the same method at DC in the 1950s, although it certainly is possible -- a close analysis of any surviving Kirby 1950s DC originals might give us some clues as to who wrote those stories.
Super Powers # 1, pg. 14
This is one of my favorite photographs of Jack and his wife. They look like movie starts to me. I wonder how different their lives would have been if they had moved to Hollywood in the 1940s. It's amazing how where you live has such a tremendous impact on your career choice. It's unlikely Kirby would ever have even considered being a comics artist if he hadn't been born in such a hotbed of publishing like NYC. This image originally appeared in Greg Theakston's The Complete Jack Kirby Vol. 2 (1997). Thanks to Mark Luebker for providing the beautiful scan.