Thanks to Tony at The Cryptic Archivist for providing a high res scan of the above photo from The Dragon #31, November 1979, which shows John Eric Holmes with his wife and son Chris talking to Tim Kask, the editor of the magazine. The photo was taken at fifth Strategists Club Banquet at Gen Con XII, August 16-19. This issue also has the distinction of including Trollshead, the first Boinger and Zereth story published in The Dragon. For more on issue #31, see this recent review on The Land of Nod blog.
Above is an advertisement for Gen Con XII from the Dragon #25, May 1979, with fantastic artwork by Kenneth Rahman aka Elladan Elrohir aka Eymoth. His tag is not to be mistaken for the EO used by Erol Otus, and can also be seen on the covers of other TSR games including Boot Hill and Divine Right (which Rahman co-designed with his brother Glenn). The same artwork, but printed in red and black was also used for cover of the convention program. A thumbnail of this can be seen on a page over at Tome of Treasures, which also gives an overview of the contents.
According to the info listed there, Holmes ran two games at Gen Con XII, "D&D For Beginners" and "D&D on Barsoom". Holmes had earlier used the title "D&D For Beginners" on his original draft of the rulebook for the Basic Set. In running the "D&D For Beginners" game, the Holmes employed an impressive game aid. Per Chris, we used "my rather large model dungeon 3'X6' ... Dad did the basic carpentry on it and together we ran "Beginners D&D" at our 2nd Gen Con".