More accurate PNG library for .NET 2.0?

Yes, I am using C # 2.0 and I need to do something with PNG files, like using a custom palette, which I can't seem to figure out how to do with System.Drawing.

Is there a third-party library (preferably GPL-compliant) that will allow more fine tuning of the PNG encoding control?

Update: Examples of what I want to do:

• Creating, editing and deleting a palette of images (deleting a palette is associated with changing the color space).
• Add or edit text snippets (iTXt, tEXt and zTXt).
• Add or edit other chunks (like bKGD and sPLT).