For Grassblades, I tend to go with something that is representative of what happens in the chapter/is the theme of the chapter, without being too obvious or spoiler-y - which is a difficult balance to strike!
... And as it is a difficult balance, I thumbnail the heck out of everything before I settle on a final design. My first chapter cover went through seven versions before I landed on the one I went with.
Examples:
This is my main character (though we can't see his face), with his right hand on the sword that is very, very important to him. There's a significance to it being his
right hand, and the scrap of cloth tied to the sword is the same colour as a piece of clothing the second main character wears, and there's a significance to the beads as well, but that hasn't come up yet. I like working non-obvious symbols into the covers, because it gives people things to wonder about, AND it gives readers the chance to go "Oh, so THAT'S what that means!" once the thing is revealed.
Part two - it's the sword again, because that's important, but there's also a lot of blood (because there is more blood in the chapter), and the rest of the items scattered around are references to things that appear in the chapter, AND new characters.
At some point, I probably will draw a cover on which my main characters can actually be seen, but my cover-designs are usually more inspired by book-covers than my comics-covers, and book-covers tend to be less focused on featuring their main characters.