All levels are by me (Connor Hawke) and my brothers (Patrick "Hawk" and Liam Hawken). As to who specifically made which level: we created these so long ago in our youth, and with Pat gone, it will take a little more time to work that out in our spare time. In some cases we're able to confirm an author because we had first drawn levels even longer ago, as young kids, with old-fashioned pen and paper, but this was not the case for many levels.
Anyway, in theory there should eventually be a sea of colors above indicating the following authors:
Bold blue - Connor
Italic blue - Connor (maybe/probably)
Bold red - Pat
Italic red - Pat (maybe/probably)
Bold green - Liam
Italic green - Liam (maybe/probably)
Normal purple - confirmed uncertain
Italic purple - other (see inline note)
* This level or a variation is featured in Challenge Go.
If you're colorblind with trouble differentiating among some of the above you can view the HTML to determine. Apologies!