Just what is going on in the illustration? The dead lizardmen and the guy cleaning his sword indicate a recent battle. Two figures in the foreground appear to be consulting what may be a map. Two people have climbed the huge demon idol and are attempting to pry loose an enormous ruby from one of the eye sockets so there are riches here to be had. The presence of a chest also seems to indicate treasure. The robed figure to the right seems to be either enlightening the warrior on the history of the evil temple or instructing hirelings on some course of action, perhaps to pile the dead lizards on the altar for a ritual or just to get them out of the way.
Since the cover wraps around to the back of the book there is more happening than first seems apparent. There are more people present and one is dragging a dead lizardman from outside the room into it. Are they trying to cover their tracks in case more enemies happen by or could more bodies be needed for the aforementioned ritual?
Did the lizardmen worship the demon of the idol or are they servants of somebody that does? Will prying the great ruby eyes from the idol result in a curse or other mishap? Did the map lead them here or is it leading to even greater danger and riches?
Below is a reproduction of front and back covers as a single piece. If you know who it is that produced this reproduction please let me know so I can edit this post to give them credit.
Thanks Dave, your art still inspires the types of questions that get the minds of DMs churning out dungeons and players excited to explore them. Rest in Peace.
EDIT: Thanks to Restless in the comments we have a link that gets us to more recreations by the artist responsible for the above rendering.