Well, all that is decided based on dynamics. With your banner your trying to make a first impression, so something with energy and is interesting to look at will, hopefully, get people excited to see more.
So the pose, for example, Mickey (letter A) is in a more dynamic pose than Fred Flintstone (letter C). Mickey has a very strong silhouette, where you can easily see what he's doing even when blacked out, he has a curved line of action, and the pose is full of life and energy. Where Fred, on the other hand, seems pretty dull in comparison. He has a very straight line of action and that's about it. Spongebob (F) also has an awesome dynamic pose on this chart. The dynamic poses are more memorable at a fleeting glance than the non-dynamic poses.
As for angle, sometime angling a shot or even forcing perspective can add more energy to the composition. It's the same reason why sometimes in movies they'll shot something at an angle (dutch angle).
This is because leading the eye around the screen/image is more interesting to look at than something in the center of focus.
Both the man and the ball are on star points in the Rule of Thirds. The viewer's eye moves from the man to the ball in a diagonal motion keeping the composition interesting, even if the subject matter might be boring. (ugh, tennis
)
Expression: well you can base that on tone or the mode you want to convey. Most of the time you see happy smiling faces, and this is because the banner is offering up a first impression to the viewers. Smiling happy expressions can be more inviting than negative expression. Even horror villainous banners can still have creepy smiles or smirks.
Basically, in general though, powerful expressions, emotions, and energy are what will leave a lasting impact on viewers. And all that come back to dynamics. It's like the art version of the power cord on a guitar.
There's a lot of physiology mumbo jumbo that plays into art and the viewer as well, like the physiology of color, but that's a lot more to explain.