I have successfully added other .dll files to other C# project this way:
Right click Reference > Add Reference > Browse > Double click the .dll file 
but Microsoft Visual Studio 2008 issues the following complaint:
A reference to ...\dll\FreeImage.dll could not be added. 
Please make sure that the file is accessible, 
and that it is a valid assembly or COM component. 
I am using .NET Framework 3.5. I believe this is a 32bit dll (it downloaded with FreeImage3151Win32) so I changed the configuration of the project to x86.
What is the correct method to add FreeImage.dll to a C# project?