FireAlpaca

FireAlpaca is a comprehensive image editing tool that offers a lot of options, functions, built-in tools, and advanced features, yet it's also completely free.

Though it's presented like a “paint” tool, its numerous features make FireAlpaca look more to me like an alternative to powerful image editors like Gimp than simply a more complex replacement for Windows default Paintbrush app. The interface is indeed pretty straightforward, intuitive, and well-structured, but advanced tools and features like the layers or the numerous brushes make this app capable of handling even complex image editing jobs. It can also be used to draw illustrations from scratch, without much effort.

All the common image editing features and functions are also available within this program. For example, filters, selection tools, and color adjusting functions are included. Unique effects can be created using the cool available brushes and pens. The watercolor is just one of the most popular ones, but you can also draw perspective views and concentration lines using the “Snap” tool.

To sum it all up, FireAlpaca is a great image editing tool that comes with a lot of cool additional features and yet it's completely free.