First, there are "rules" about how to do this. I will tell you about them, but since I don't like design "rules" I will also show you how to break them!
One easy way to help you arrange many pictures together is to use craft paper (or any paper really) and trace your frames on the paper, cut them out and mark on the paper where the nail will go with that frame. This is your template. Cut out all of templates and use painter's tape to attach them to the wall you're working with. You can move them around, see how they work together, what fits well and how it will look in the end. The bonus is you've already marked the nail position, so you can hammer the nail in right over the paper, tear the paper down and hang your picture. Instant wall of well-placed pictures!
Here are some ideas to help you arrange pictures above stairs. I've attached two pictures to help me articulate my ideas.
The most pleasing arrangement is to follow the line of the angle up the stairs and use that as your base line for your pictures. Make sense?
Take a look at the photo to the left with the two pieces of art. Simple and clean and if you drew a diagonal line connecting the bottom of the frames it would run parallel to the staircase. Sounds much more complicated than it is.
Larger, "heavier" frames usually look best near the bottom of the arrangement, as they help weigh it down.
While I think this looks great, there are other ways to hang pictures above stairs—that look just as great.
This picture, with the black-framed art, doesn't follow the angle of the steps, but plays with the wainscoting and shelves, stacking the pictures along those lines and creating great visual interest with the placement and contrast of the frames with the white wainscoting.
The frames unify the look and the "stepped up" arrangement works with the steps.
I love it.
Try different things, do what looks good to you and showcases your pictures in their best light. Be brave, there's no "right" or "wrong" and anything you don't like is easily fixed!