Wow! Spazazo, that was superfast! And what a fun drawing! :-)
You could do a really quick shade-job just to see what happens. Don’t labor over it – just knock in some values and see what happens. And don’t be afraid to ruin it. You have to ruin a lot of art to learn the ropes.
Have you got a way to scale that down? Your image is a lot larger than it needs to be.