They actually name the manga the sketch is from in the "Staff" Section of the website. http://www.katawa-shoujo.com/staff.php
Considering they're a team that gets together from all over the internet, they have done an INCREDIBLE job so far. This is the kind of project that gets a group recognized by bigger companies, it's a shame for that reason that visual novel production doesn't have any kind of establishment in the U.S. :/
I'm also kind of shocked no one has posted any screens yet.
Rin's my favorite so far. :3 I'm not posting anything plot-related so you all have more reason to go play the demo. Now!