Unfortunately, your screenshot is not showing up. Could you post a link to it instead?
The StartButton has to be a specific size to look right, especially on the Classic Taskbar, and the combination of sizing margins and painting method can affect its appearance. Also unfortunately, SkinStudio's Preview doesn't give a true image of *all* controls as they will appear in the applied skin, the StartButton being one of them. As GreenReaper has said, "SKS is hopeful like that."
Cheers,
Daiwa