I have always been a fan of hanging art in the bathroom.
It’s a space that is often neglected, artistically speaking, however the right art can really shift the mood of the room. Have fun with it, be cheeky, sexy, sensual, playful, whatever your heart desires, since it is a small space, the larger the photo/art the better. Statement pieces give the illusion of something bigger, even in smaller spaces they can act as windows in restrooms that have none. Expansive shots of nature will open up the room.
Regardless of how you choose to incorporate art into your space, remember that art elevates the soul and it is too important not to share, even in the lavatory. -tM