I've been hovering around the top 30 in the Graphics category with anywhere between 1-5 sales/day of the $0.99 app Quick Resizer[1]. It's not that hard in certain categories.
Notably, this article fails to mention that this is for only the graphics chart, and the selection of graphics apps fairly limited. Outside of Serif Labs, Bohemian Coding and Pixelmator, most of the apps are fairly one-note like Redacted. I don't think desktop users are of the same mindset for one-off applications outside of small utilities like SnapRuler or Colorsnapper.
[1] https://itunes.apple.com/us/app/quick-resizer/id498486288?mt...