Windows Live Gallery - Developer center: "Windows Live Writer extensions
Extensions give people the ability to insert, edit, and publish new types of content.
Before uploading extensions, read the Windows Live Writer Software Development Kit (SDK) and review the following checklist:

For Content Plugin Extensions
1) Content Plugins must be built using .NET Framework 1.1 or 2.0.
2) Content Plugins must be packaged and submitted as a Microsoft Installer (*.msi).
3) If you use .NET Framework 2.0, the installer must link to the .NET 2.0 download site or use the Visual Studio 2005 Bootstrapper. The Bootstrapper detects and downloads the correct .NET Framework.
4) Your installer must copy the assembly to the 'Plug-ins' sub-directory of the Windows Live Writer installation directory."