When you develop custom functoids in BizTalk Server 2006, one of the steps you have to do is create icons to represent them in the mapper.
These functoids must be created as 16x16 BMP icons, and inserted/embeded in a Resources file. You can do this directly in Visual Studio, by opening the Resx file and adding an Image file, or (my preferred way) by using Lutz' Resourcer for .NET .
You can start by creating an icon in Visual Studio's editor, which I then screenshot into Paint.Net and save as a 16x16 BMP to later insert into the resources file.
Another "detail" that shouldn't be forgotten is updating the "Custom Tool Namespace", on the .resx file. Also remember that you have to restart Visual Studio if you want to have the icons/dlls update, as it caches the functoid assembly from its path at <drive>:\Program Files\Microsoft BizTalk Server 2006\Developer Tools\Mapper Extensions.
Using a background color for the Functoid icons is good practice, to avoid confusing yours with the built-in ones.