You can read all about HD Photo here: http://en.wikipedia.org/wiki/HD_Photo
NOTE: This is a BETA release.
1) Copy HDPhoto.dll to C:/Program Files/Paint.NET/FileTypes
2) Restart Paint.NET (if it was already started)
If you installed to a place other than "C:/Program Files/Paint.NET" then substitute the appropriate directory name.
v0.1, April 5th -- Initial release. Expires on June 1st, 2007.
v0.2, April 28th 2007 -- Added ability to open images that are not 32-bit BGRA. Also added ability to save in 24-bit format. This version now expires on July 1st, 2007.
v0.2 (again), June 16th, 2007 -- Recompiled and re-uploaded. Expiration date is now September 1st, 2007, and also will expire if Paint.NET reports that it is version 3.20 or newer. July 4th -- reuploaded version that should work now.
v0.3 -- September 8th, 2007 -- Recompiled and re-uploaded. Removed date-based expiration in favor of version-based expiration. This plugin will cease working if Paint.NET is upgraded to version 3.20 or newer (which doesn't exist yet -- the plan is simply to have the plugin integrated by then).
v0.4 -- December 14th, 2007 -- New release that works w/ Paint.NET v3.20. Version expiration now set so that it won't expire until Paint.NET v4.0.