Have you ever tried to create a transparent GIF with GDI+? Seems like it should be easy since there's such a thing as Color.Transparent, but interestingly enough the GIF encoder that's included with the .NET framework doesn't correctly identify Color.Transparent! But there is a way to get the job done by using unsafe code blocks to directly access the memory where the bitmap is held. Here is a fun WinForms sample that allows you to read in any GIF file and set any color in the palette to be transparent:
It will load in any GIF image, and as you mouse over the image you can click on the color you want to be transparent. You'll know what color you're selecting because the whole background of the form will turn that color. Once you click, a Save Dialog box opens and your new GIF with transparency is saved.
You can download the source code to the solution here. It was made in .NET 1.1, but will convert easily to 2.0.