解决起来还真是很麻烦。lz看看下面的讨论:
Transparent controls in WinForms are transparent relative to their parent, not to other controls. Transparency in WinForms is more akin to camouflage than true transparency. A transparent control doesn’t actually let you see the control behind it through the form. It asks its parent to draw its own background on the "transparent" control. This is why a transparent control shows the form behind it, but covers up any other controls.
来自下面的讨论
http://stackoverflow.com/questions/12138892/picturebox-with-transparent-background
这里有些解决方法:
http://stackoverflow.com/questions/4144371/a-picturebox-problem