There are many composition rules that you can keep in mind when creating your cover such as the rule of thirds and the golden ratio. You can work with contrast in terms of color, texture and size.
The most important part of your cover is informing the reader about what they're getting into. Having a vague and less detailed cover may work if the story is a mystery, but outside of that, a possible reader may turn your story away.
I like the two cover you have already but, at least for the completed one, there is a flatness to it. The art is good, but you could do more in the background. It's hard to tell what the story is going to be about, though I can make some assumptions. The story is likely a lighthearted comedy about a boy and his hamster. Or maybe guinea pig, you know how confusing that can be. I'm not saying you should redo your cover art, but if you do, I think what you have already have is a good starting place. I don't want to speak on the uncompleted cover, because there's probably going to be more to it than what I can see now.
For posters. . . Oh this is tough one. The funny thing is that I do have some poster art in the works right now but, I'm not completely sure about what I'm making. I don't want to given you any uninformed advice.
Okay, so for the title of my present comic, I made it up in high school and never let go of it. But for another story I'm working on, I got some great advice from an author youtuber: Jenna Moreci (i reccommend getting writing advice from her) When coming up with a title, first start with keywords that are staples of the genre. If that doesn't work, take reoccurring words and symbols from your story, list them and try and make a title from those. Using your main characters as a title name also is a good option.
I'm in the middle of redrawing my first chapter and I'm going to rework the cover too. This is my cover right now.
The new cover will look similar, but the point is to inform a possible reader about what they're getting into.