Paint.NET is image and photo manipulation software designed to be used on computers that run Windows 2000, XP, or Server 2003.
It has the following features and you can download the c# source code to look at it under the hood.
Graphic Design
- Layers
- Near unlimited history (limited by disk space)
- Various Brushes
- Many special effects and adjustments
Photography
- Auto-Levels and manual Levels adjustments
- High quality bicubic and super sampling image resizing
- Magic Wand selection tool
- Clone Stamp tool
- Red Eye Removal
- Ability to acquire pictures from a scanner or camera directly into Paint.NET (Windows XP/2003 only)
Web Design
- Up to 3200% zoom, or as low as 1%. Grid mode is available for 400% and larger zoom modes.
- Control over JPEG quality along with preview of resultant file size
- PNG transparency support
Developer
- Extensively optimized for multiple threads: systems with multiple CPU's or cores will exhibit dramatic performance gains
- Full source code provided
- Extensible effect plugin system
Read more here.