You can hang anything that you like! I love the look of a painted canvas as a way to bring in a lot of color!
In my master bath, there is a towel rack with a heart shaped shell mirror over it. I have a picture over the toilet. And a few more sea shell items.
I have animal portraits (XS, painted) owls frogs jaguar
If you're on a tight budget, head to the dollar store. I've found lovely canvas art and other framed pieces there. Pick a theme for your bathroom, then choose pieces that match. For example, my ensuite has a botanical theme, so I chose floral art. You can also easily make your own art using dollar store frames. Try something similar to the butterfly art in this article:
Or the botanicals at the end of this article: http://www.dollarstorestyle.com/picturethepossibilities.html made using sections of a wallpaper border.
The craft stores - Michaels, Hobby Lobby, even Christmas Tree Shoppe all have really nice prints and more and more decorative prints.