The above piece was penned in 1980 by Jim Sherman--perhaps better known for his work in comics including Superboy and the Legion of Superheroes.
Before continuing I wanted to answer a few objections to this sort of picture that I find annoying:
No one actually looks like that. Of course they do. Come to my gym sometime. The weight room looks like a Frazetta painting without the jungle ferns. I'll introduce you to a hedge fund manager who is the spitting image of that guy.
It's unrealistic. It's true that an odd thing about this particular illustration is that it features a sandaled, Conan style warrior in a Northern European fantasy milieux. Note the pine trees and the Germanic Cinderella's Castle-style dwelling in the background. But consider this: I often wear light clothing in my condo, even in winter. I hate sweaters. They constrain me. Our friend probably had similar tastes. But out of a window on a cold stormy afternoon, he noticed the dragon padding down the cobblestone road with dragonish murder in his eyes. Our friend quickly grabbed his helmet, sword and shin guards and then slinked from pine tree to pine tree, finally surprising the dragon from behind. He posed for a picture before sprinting back. Brrrr...
Enough joking around. The above painting illustrates the pulp/comic book aspect of old school fantasy art. As we shall see, it was only one strand of old school art, but it was an important strand. Since 3.5e it's been pretty much gone from TSR/WOTC/Hasbro products and is nowhere to be found in 5e.
That's a shame.
Curiously, Hasbro currently owns the rights to DragonQuest, which while never really catching on in a major way apparently still has a small group of loyal followers. It went through a number of editions, the first two in 1980 and 1982 by the fading war-game company SPI, vainly trying to hitch a ride on the roleplaying game steamroller. SPI was then purchased by TSR which put out a 3rd edition in 1989.
On an interesting side note, if you don't like the pine trees, above, the layout people for the 2nd edition probably agreed with you. For their cover they used the Conan-like figure and the severed dragon head but whited-out everything else. You can see it here in Grognardia's useful retrospective post on the game. This is the cover and edition that many of us remember. I have it in a box somewhere.
I picked the piece partly because I like it. Yes, it's cheesy and silly--especially that grin. But I think it works. I also wanted to emphasize the obvious point that 'old school' sensibilities in fantasy art do not merely apply to products put out by early TSR.
But we can find many similar efforts in official Dungeons & Dragons publications. Here's an illustration from the very first issue of The Dragon from an article titled 'Hints for D&D Judges, Part 2: Wilderness':
It appears to be a Dave Sutherland piece. So, to the Tom Wham 'funny-stuff' and the David Trampier sort of 'Dungeon-noir' drawings, we might add the slightly more pulpy and comic book-like tone of Sutherland. One of these Fridays we'll feature one of his illustrations.
Finally, this post is a kind of stand-in for the planned second installment on the Eldritch Wizardry cover. Before I sidetracked everything by first talking about clothing, I wanted to make a second point that the Eldritch Wizardry cover represented the lost pulpy element of old school art. Practically every literary source listed by Gary Gygax in his famous Appendix N (the link is to one of Martin Ralya's useful blogs), had one or more pulpy covers attached to it, either in the magazines of the 30's and 40's or the mass-market paperbacks of the 50's, 60's and later. One could say that some of it was sexist, borderline pornographic or just plain stupid or bad. But there was also a huge amount of good in it, and Its studied abandonment by the current market leading roleplaying game is a net loss.