superimpose two picture boxes c# ?

There's a good article here explaining one way of doing it:
http://www.codeproject.com/Articles/25048/How-to-Use-Transparent-Images-and-Labels-in-Window
The article states that Windows Forms controls don't support true transparency, but shows how to use a mix of PictureBox, Panel, Label and GDI+ drawing to achieve a solution.

You can also accomplish image-over-image transparent blending using other controls like this sample project shows:
http://support.leadtools.com/CS/forums/44070/ShowPost.aspx