Transparent buttons, C# -> Windows 7

I have an issue, and wonder if anyone has any thoughts on how to resolve it.  I've developed an application in Visual Studio 2005, which uses a lot of transparent buttons.  (An underlying jpg gives the context to the button, so there's not even a text label.)  When I run the application in Windows XP, it looks great.  When the same application is run in Windows 7, the buttons are NOT transparent, but this pearly white color.  When I open the sln file in a version of VS running on Win 7, the buttons have this same pearly white appearance, so it seems to be an issue with Win 7, rather then VS.  Any thougts on how to make these buttons transparent?

Thanks in advance for any help you can offer.