Most commercial art isn't of knights/space battles... Quite a lot of it is used to sell some product, not the art itself. 

You think you need a degree. Here’s your 5 minute degree in commercial art. 

Take an old Frazetta Conan cover, the kind where Conan staring down a giant tiger. There's usually some nearly naked chick twined around his calf, staring lustily at his bulging biceps. Your assignment will be to change it to commercial art.

 

Dress the girl; after all we have to at least pretend we're appealing to the higher brain functions. She needs to still be naked, just clothed. A good 3/4 profile that silhouettes the curve of her breasts works well. A polka dot or flower print dress with dots that just happen to be placed at the correct spots can also suggest what’s hidden. Rather than show her thighs outright give her legs a good wide stance. Put her in a mid length skirt but curl the edge up like a gust of wind is about to blow the whole thing up. A promise is better than the real thing. Your brain can imagine a hundred possibilities of what will happen next. If you actually show it you will have only one result and disappoint the 99% of the ad viewers that expected something else.

 

Our heroine is like Jessica Rabbit. “I’m not a bad girl; I’m just drawn that way!” The girl is ultimately you/the ad viewer. You’re not a slut! (even if you are.)  It is the natural order of things that the most deserving be endowed with an over abundance of grace, sex appeal, money, or whatever you are trying to sell in the ad. Nobles’ oblige requires you never come right out and say this however. It’s a twist of divine fate not extensive cosmetic surgery they look like Pam Anderson. Sultry vamps never look sultry, they always look scared, concerned, contemplative, or like their oldest bestest friend who just happens to be a priest just walked through the door.

 

Replace Conan with your product. The product must still be heroic. The glistening sweat of his biceps needs to become the gleam of the ultra whiter-than-white enamel finish of the new FrigereaX ultra massive side by side refrigerator. His shining broadsword becomes the easy-grasp chrome door handle. His massive belt buckle, the space-saving side-by-side water and ice in the door. His golden headband becomes a light source centered on the background behind the top of the fridge. The edges of the piece should get progressively darker, suggesting the only security lies in the center with the Hero/fridge. Your product should still tower above our heroine to keep the savage beast at bay.

 

The tiger represents loss. As Zig Ziglar said, “The fear of loss outweighs the desire for gain.” Tell someone that a product will make them prettier, healthier, anythingier.. and they’ll ignore you. Tell them, “Gladys down the street is got it first! Now she looks better than you.” and they just have to knock her off her throne. The tiger becomes, “them! The Smith’s trying to keep up with your Jones.” Put them below you/the non-naked girl and the product/hero. They can be as subtle as vague eyes in the dark or shadows reaching toward the hero. They could be physically absent but suggested in the ad copy. Like the skirt they should never be too concrete or draw attention from the hero. A vague threat can be filled in with Gladys (I’m gonna’ take Tom to the dance), Momma (Stand up straight! You’ll never be good enough if you don’t...), or whatever the viewer is most afraid of.

 

So here’s your 5 minute art degree diploma. The rest is just composition and perspective that you already know.