I'm glad you liked the art. It's from the Premium Crayon And Kazoo Asset Pack by Damon L. Wakes. I know him from working in previous game jams and he makes a lot of interesting content! I believe they were drawn by hand and scanned in. Oh and you can totally download the assets yourself for FREE at the moment, he even has a gamejam going on that strictly uses the assets until the end of the month!
(The credits are listed at the bottom of the SLIME TOSSER page)