well, he already seems pretty spock-ish. You may want to compare against a reference picture to know whether the ear should appear behind the skull or not. Maybe you want to draw a vulcan-salute hand rather than a "hi!" text balloon, which sounds out of place given Spock's habits.
You should better try to save your png at 1:1 ... there's some odd scaling effect, with edges between pixels being blurry. Having a clean picture definitely helps to get edits.