Good question. My husband draws and paints a lot. I took a picture of one of his favorite characters (my favorite painting), and set that as his profile pic. I'm sure at least some of those are self-made. Others might be things, from various sources, that were put together in a paint program.
Most of the people use Photoshop nowadays. They could be picking cartoon pictures and editing the features using Photoshop. In the program, there are different colors that someone can use to change the text image.