Martin Gately tells me that the art for the Wolverine stamp is from the first page of Giant-Size X-Men #1. This comic was pencilled and inked by Dave Cockrum…so that credit's right.
John Kowalchuk recognized the source of the Spider-Man portrait. It's from the cover of the book, Marvel: Five Fabulous
Decades of the World's Greatest Comics by Les Daniels and it was drawn by John Romita. So that one's right, too.