I prefer adding quality rather than quantity. Writing new articles or section is nothing for me, but the more is correcting typo's, rephrasing unclear sentences, and more of such copy-editing tasks. Some say I have a hawk's eye for detecting small errors that may be easily overlooked, such as typo's, letter swaps, double spaces and repeated words. I might be considered a WikiGnome.
Mostly I edit in the Mathematics sections. My largest interest is in finite geometry, but I take the effort to correct mistakes in all articles I read.
Pictures are generally useful additions to an article: they don't only make us better understand (mathematical) concepts, they make it possible to seize the essentials of an idea in a blink. That's why I plan to introduce pictures where they would be useful, as well illustrations of mathematics-related concepts, as public domain photographs.
Mathematical articles needing images: here and here.
I add photos of living persons with their permission, especially mathematicians, quizzing champions and other famous people I meet.