Free hand your own artwork. It doesn't matter if it sucks, the kid won't know. It'll love it. Plus mom and dad gets to be goofy for a few minutes being artistic. Paint over it when the kid outgrows it.
We did some cool silhouettes on my kid's walls. Google "____ silhouette" and find the image you like. Set up a PC projector and put the image on the wall. Trace with pencil. Get some black paint & paint in the shape. Blammo - free image on the wall in the size/shape you want. My 15 y/o daughter did them and they look great. Just look for images that don't have too much complexity.
We just bought some stickers on aliexpress. They're pretty nice and cost like 15$. Ikea has some cool ones too.
It doesn't matter. By the time he's 5 you won't be able to find the floor, and his dirty undies will be stuck to the walls..
