How to make Visual Studio toolbox icon have transparent background?

During my web developer career, I wrote about a dozen of ASP.NET server controls. However, transparent background in the Visual Studio toolbox was always a mystery for me. Because of that I usually used icons without transparent background. I noticed that DevExpress controls have those kinds of icons, so I used Reflector to browse their resources and investigated this issue. To make a long story short, here are an icon with transparent background requirements:

Image Format

If you want transparent background you need to use 16x16 8-bit BMP. You can use other types of images jpeg or png, but transparent background works only with BMP. This is very strange since BMP doesn’t have an alpha channel, so no build-in transparency.

Transparency Pixel

Select neutral to your icon color and color with it all transparent places. DevExpress usually uses Magenta (#ff00ff) for this purpose. To make a selected color transparent you need to color a bottom left corner pixel of your icon with it.