Oh, boy, do I love art! We all do if it's to our liking. What I do find not so attractive is artwork that doesn't 'fit' the wall. That is, the art is tiny amongst a huge expanse of wall, something that's not at all pleasing. Ideally, you want your art to take most of the wall space on either side of it.
Here's a look at two types of wall art that will go well with your green and black bedding. The large tree wall decal can be applied upon a green wall for a like contrast with your bedding. The decal will look fabulous behind your bed, as it's quite sizable and will command the attention it so deserves. Wall decals are a great option, as they're easy to apply, removable, reusable and very budget-friendly. The decals come in tons of colors and subjects; you'll find one you just 'gotta have', I'm sure.
Another option, is small prints like the one, here on the right. Be sure to hang them on a wall with just a few to several inches left on each side (left and right). You'll really dilute its impact with it getting 'lost' in a sea of wall expanse. Keep your artwork lovely and give is the attention as a strong, visual piece by not drowning in out with wall space. I would pair the smaller print up with another of its ilk--or four if your space permits, to create a nice gallery effect.