Today I have released the first (read: very alpha) version of a plugin I’m making for Paint.NET. It allows you to load XNBs containing compiled Texture2D resources and edit them at your pleasure. Current supported data format is uncompressed assets compiled for Windows or Windows Phone (Xbox 360 uses a different endian and is on the works), with pixel formats Color, Bgr565, Bgra4444 and Bgra5551.
Future additions will include the ability to load the full mipmap chain, DXT-compressed textures, Xbox assets and resaving your work in a new XNB. So, if you want to try it, go to the project’s Downloads page at CodePlex, or browse the lastest changesets.