While clusters of small frames and mirrors of varying styles and sizes can be very pretty, I do love what a large piece of artwork can do to a room. Paintings and photographs with bold colors and beautiful compositions can function almost like panels of dramatic wallpaper, which is especially welcome in more neutral rooms.
Of course, your choice of art should be governed primarily by your love of the art itself, how it speaks to you, what emotions and reactions it evokes (or even the value or cultural significance of the piece). But it doesn't hurt when a piece of art also manages to elevate the beauty of the surrounding room, drawing out colors and textures that may otherwise be diluted (the color of a book's cover or the curves of a vase, for example). In each of these rooms, the artwork would be lovely on its own but is even lovelier for the way it interacts and elevates the rooms in which it hangs.
6. Chicago Tribune via Stephen's Bachelor Pad: A Lesson in Using Color Wisely
7. English Muse.
8. Because I'm Addicted.
9. Architectural Digest via Everything LEB.
Images: As credited above.