I got a Cricut Maker recently, and I’m determined to use Linux to drive it! Besides, I don’t have windows installed anywhere so I don’t really have a choice.
A couple of people mentioned GIMP above, but that’s not really the right solution; you need a tool that deals in vectors instead of pixels. The Linux standard for that is a program called Inkscape. It takes a bit of getting used to, but it produces SVG images which you can then import into the Cricut mobile app (use the “Upload” button in the app) and send to your printer.
I’ve done a few prints this way and it been working pretty well for me!