This is a mural that I painted in my son's room nearly a year ago.
First, I took tons of pictures of him blowing bubbles. I used a data projector to project one of them onto the wall, and I traced his silhouette. Then, I added the circles in a variety of sizes to be the bubbles. I found images that represent his specific interests and used those in some of the bubbles: breakdancers, cars, musical instruments and a soccer player.
You can find lots of good images to use for murals when you search "silhouette" or "vector" along with subject you are trying to find. Many of these are copyright free; just add that to your search.