It's hard for me to say what would happen if I didn't go to art school. It wasn't that I learned any specific painting or drafting skills at school that I felt I couldn't have taught myself. However there is something quintessentially unique and important that you gain by immersing yourself in ascholastic and creative universe, and being held to certain academic standards while being surrounded by artists of varying disciplines.
If what you want to do is make artwork for bands, you have to love doing it because there is almost no money in it. In order to start doing it, you just have to put yourself out there, work for bands you love and for as little as possible to start, if not free, that's what I did for years.
It can be difficult to mediate a compromise between what I have in my head and what the musician has in mind, which is often 180° different when it comes to the finished product, so it requires that element of trust from somewhere. The point I make to them is "You've seen what I do, so just trust me and we will come up with something exciting."
It's important for me to have different tiers of value for the art. Some of the silk-screens are really affordable, but I do have some high-end silk-screens that are several color layers and a little more expensive, and then I have the paintings that can get way up there.
If you want to create something that's worth doing you have to self-edit from the get-go. You really must be careful and selective with whom you work, you must constantly ask yourself the hard questions about your art, and you must set a nearly unattainable standard for yourself.