|Well, I don't see the problem with Yamada-kun to 7-nin no Majo cover, since it's the newest volume. True, for now he only shows up occasionally, but it seems he will show up more in the future. For ongoing series, usually the image will be updated once the new volume is out.
Maybe in the image guidelines, have a recommendation that if the image shows characters, that it include a main character (or in some other way accurately represents the series).
And what if the cover design for the said series all don't have the main characters on them? Accurately represents the series... it's somewhat vague too, since each manga have their own design (that I believe) well thought by the publishers. And while they only show secondary characters, I don't think it's misleading.
Let's see if I never read Yamada-kun to 7-nin no Majo before and just now decided to read it, what I see maybe something like this:
a. The image shown is the newest volume (vol.7)
b. Since it's already vol.7, the guy shown probably not the MC, but a side characters. But since he is on the cover, he might be an important character (for the whole series or just focus of the volume).
c. The art is nice and clean. The setting is in school. It might have something to do with some fantasy elements (from feather-pen and the background, reminds me a lot of Harry Potter. Hey it might be something like Harry Potter)
Whether I decided to read it or not, it depends on my preference (if I only see the cover, not the comments or summary). If I like fantasy, school setting, the art, I might try reading this. That's just my two cents.