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)
How To Create a Transparent Picture For Office CommandBar Buttons (MSDN KB Article)
How To Set the Mask and Picture Properties for Office XP CommandBars (MSDN KB Article)
Given that Visual Studio borrowed its commandbars from Office, it’s no wonder it has suffered the same problems.