The art is hard to pin down. It seems like it was done by one artist, but I can't really tell who.
Update: Thanks to one of my readers, P, who uncovered the identity of the artist... Paty Cockrum!
"No, it is not Dave [Cockrum]'s, although it does have some of Dave's characters in it. It is a montage of characters that I put together for, I believe, a jigsaw puzzle. that was one of the things I did at marvel...early merchandising art, with poses gleaned from the books, usually, although some art was done specifically for some projects. But I put this one together... and may have even drawn it. All I remember is that I had to fit in all these characters, taking into consideration all the costume colors and such to give us a decent jigsaw puzzle...LOLsheesh! they told me which heroes they wanted and I forget how many but i believe it was over a hundred! LOL"
- Paty Cockrum
And here it is on the wall in my comic book room. I've had it seen 1983 and it's missing the bottom right piece as it's moved along with me wherever I've gone!