The cover image is via Tome of Treasures. The ad is from Dragon #33 (January, 1980). I don’t have any of the interior art, unfortunately, but the art credits are as follows:
John Barnes: April and January (1981).
Dale Carlson: January (1980), May, September, and December.
Elladan Ellrohir (real name: Kenneth Rahman): February and November. Rahman, apart from artwork under several different guises, co-designed TSR’s Divine Right and Knights of Camelot games.
Phil Foglio: June.
Bill Hannan: front cover (from Dragon #1), July, and October.
Dean Morrisey: March and August.
The incomparable Darlene did the titles and title illustrations. With due respect to the other artists, that’s what I want to see most.
I thought the first Dragon calendar came out in ’79, but I can’t seem to find any evidence.
2 Warps to Neptune 
I think there was a 1979 calendar. I seem to remember the cover picture was of a bunch of heroes, in the middle of some vast arena, with a bunch of dragons flying in through a keyhole shape in the far wall. I’ll have to check some early The Dragon mags to see if I can find some mention of it.