There are some wonderful Avengers covers that Jack Kirby did during his mid-late 70s Marvel stint. Here's one of my favorites: Marvel Treasury Edition #7, from 1975.
I like this cover as it features my favorite classic Avengers, running straight at the viewer. Thor is front and center in a timeless pose, swinging the Uru hammer above his head. Cap looks cool to the left and on the right you have Henry Pym as Giant-Man (blue costume).
If that wasn't enough, Kirby also did the back cover with a different Avengers lineup. The Hulk makes an appearance and Henry Pym is in the red Giant-Man costume--and does double duty as YellowJacket! This pinup must have foretold Avengers Forever. :-) You can see some retouching going on here--I believe that the Beast, Vision and Iron Man were slightly changed to make them look more like the contemporary versions. Great stuff. Nuff Said!
Update: Comments from my old MT blog...