Because OSX is basically Unix underneath, you can use GIMP on a Mac.
https://www.gimp.org/downloads/
GIMP is free-as-in-speech as well as free-as-in-beer, offering functionality somewhere between Paint.NET and Photoshop. It can be a little bit intimidating at first to get set up for true pixelbashing in the Shipbucket style, but I'd be happy to offer some setup advice over PM if you're interested (one of my personal machines is a Mac).