One of my favorite illustrations, in all of D&D, isn't a great illustration because of the amazing quality of the work, or the incredible composition. It is this illustration of alignment in the Moldvay Basic D&D book:
The reason it is one of my favorite illustrations is how well it conveys the concept of alignment in absolutely unambiguous clear terms.
I don't need to interpret the art for anyone, because the message is so clear.
There are so many more amazing pieces of old school art I don't even know where to begin, but this link is perhaps one of the best collections I've seen.