How to implement a splash-screen using an image with alpha channels transparency/opacity in WinForms?
Take a look at Per Pixel Alpha Blend in C#
Depends on what kind of shape your image is. Is it something you can draw using code? If so you could set the forms draw region to a shape, like this:
yourForm.Region = new Region(someShape);
The shape can for example be a polygon with the same shape as your image.
来源：https://stackoverflow.com/questions/851738/splash-screen-with-alpha-channels-transparency-opacity-in-winforms