Humans are visual learners. 
Rather than face a wall of text, images help to lighten the load and when used well, makes lessons easier to absorb. Also, given the popularity of comics as a medium of learning, what better way to keep your readers interested in your lessons by incorporating light humor and popular illustration into it?
Since Mar 2014, CDS has been illustrating for Nihongo Master to help users better learn Japanese. Illustration go beyond just the basic cute humans but also more everyday scenes such as bus stops, computers and even the occasional cockroaches (brrrr). 
It probably helps a lot that we understand some Japanese ourselves and can add to the lesson with our own intepretation of both the intended lesson and the brief. For example, what to say when you make a stupid mistake.
Other than illustrations, we also did manga for their various lessons. The script and the character designs were pre-provided but everything else was up to CDS to get the story across. Of course, not neglecting putting our own little touches to the manga. 
As of Lesson I41, Ricky Li took over the illustration duties from Angela Tan for Nihongo Master. He brings with him a simplier, humorous take on the brief. 
These are user avatars that was designed by Angela for Nihongo Master
It is not always just black & white illustrations. We occasionally draw in colors for them! Generally for marketing key visuals.
Image from Nihongo Shark's review of Nihongo Master:
Want to understand the manga? Why not start by going to Nihongo Master where you can learn Japanese with a touch of light humor!
Accounts Management: KC Ng
Illustrations and Comics till I40: Angela Tan
Illustrations and Comics from I41 to I100: Ricky Li. Assisted by KC Ng, Antonio Low, Chew Hui Wen and Chelsea Low