24 Hours with Jack Kirby: Hour 1

Jack Kirby (R) and Stan Lee (L) in their first pictured appearance in the modern Marvel Universe, on the cover of Fantastic Four #10 (January 1963), art by Jack Kirby and Dick Ayers
(Click image to see the whole cover)

Their appearance in the comic concerns Doctor Doom strongarming Stan 'n' Jack to betraying the FF:



(You can read all about it at your local library in this post about appearances by the Marvel Bullpen in Fantastic Four comics!)