If you've bought it, just install it through Steam, it works on Linux and is the latest version. This one is way out of date and can't go full screen. But I guess it's OK as a quick and free fix straight from the Software Manager.
Yes the program is amazing, this package just isn't. First off, this is an older version (1.0.9 and not being updated, current actual release is 1.2.6 on their website and being updated even for Linux). Secondly, for some reason I experience mouse lag in this version, while I don't experience that in the official trial of 1.2.6
Coming from someone who's tried MS Paint, Graphics Gale, Paint.NET, and Photoshop, this is the best pixel art editor I've ever used! The latest version is commercial, but I still highly recommend it!