I have found today three links on the web about issues with transparency in custom pictures of buttons that Office add-ins have suffered too:

Alpha channels, masks, color keys, oh my! (By Eric Faller about the Office Ribbon)
http://blogs.msdn.com/jensenh/archive/2006/11/27/ribbonx-image-faq.aspx

How To Create a Transparent Picture For Office CommandBar Buttons (MSDN KB Article)
http://support.microsoft.com/kb/288771

How To Set the Mask and Picture Properties for Office XP CommandBars (MSDN KB Article)
http://support.microsoft.com/kb/286460

Given that Visual Studio borrowed its commandbars from Office, it’s no wonder it has suffered the same problems.