Identify slide master from powerpoint

Hi,
I have created an application with C# .net and powerpoint2007 (vsto addin), i am using code

Presntation.SlideMaster.Background.Fill.UserPicture(PictureFileName); 

to set background of SlideMaster, but its apply only the first SlideMaster in a presentation. when i create 2 or more SlideMaster in a single presentation , i need to select the current SlideMaster and apply the picture as background.How can i locate selected SlideMaster from a presenation and set background using c# code..
Pls help me.