Though my personal laptop is Windows, I work on a Mac during the daytime and so i might be able to help. When it comes to macs, there are a few free programs that are similar to paint. One's called paintbrush, which is really only good on older OS as it's somewhat outdated. A site I found called acorn has a newer program that's similar to paint, it costs money but there's a free trial that you could use for the jam: https://flyingmeat.com/acorn/. I've also heard something about a painting tool within safari, but I don't know much about it. Otherwise, as Yukon said, you could easily use a program like Gimp to create the art as long as it has an ms paint feel, as long as you follow the basic ms paint rules. Hope that helps!