What do you think the classes of the characters on the box art are? I tend to do things how I would see the "canon" being but I can't really see what class they're supposed to be.

depends on which game your speakin of

If You're talking about DOS:EE There are two posters, on both of them there is Fighter type (probably knight or avenger) in Orc Armor, and an Archer in some type of leather armor that i coud not find in game (only one having "wings" on the shouder is the starting avenger/knight armor). Though for reasons of gameplay i always use custom made chars created out from knight+assasin where knight becomes a 1h+shield suer focused on tanking and assasin becmes Earth+fire caster (to complement with Jahan and Wolgraff), and these 4 are making a perfect party for me. Though had one completion of game with Archer+knight dual lone wolf and it was real fun but challenging (due to fact that i was playing veteran...).

Box art and trailers are often made long before the game is completed. What I find more annoying is some games where the cut scenes depict things that are not in the game, like a piece of armour that was removed during development.