Along with Dave Trampier, there is one other TITAN of the old school D&D art, and that is the incredible Russ Nicholson, who brought to life the entirety of the Fiend Folio as well as gobs of other art for games throughout his career. While the original Monster Manual’s art was a huge inspiration, the Fiend Folio took both the monster designs and art to a new level.
Unlike Trampier, who quit doing any art in the early 80’s (and missed the MTG cash cow that he would have fit into perfectly), Russ kept on doing art for OSR modules, DCC and other games his entire career. RIP Russ!