Pinta is a free, open source drawing/editing program modeled after Paint.NET. Its goal is to provide users with a simple yet powerful way to draw and manipulate images on Linux, Mac, and Windows.
Pinta requires both .NET 4 and Gtk# installed (on Windows), or Mono (for Mac), which is why I not am going to test it. NET, yes, Gtk, no! But that is just my opinion about Gtk.
The reason I am telling about Pinta is the extensions. Pinta is really made with one special feature in mind: it must be as easy as possible for coders to write extensions for Pinta. But Visual Studio (or MonoDevelop) is necessary. The page >Getting Started Writing an Extension - Pinta
< tells step by step how to do.http://pinta-project.com/