This article tells you how to make a bitmap of a picturebox with transparent background
If you search for a transparent method in the picturebox clas, you will see that the method do not exist. So there must be another way to make the background of a bitmap transparent. Have a look in the bitmap class and you will find a transparent method "MakeTransparent". You must first convert your image to a bitmap and then select the background (transparent) color. After that, make the background color transparent and finally convert it back to the image of the control.
1) Convert your image to a bitmap
Bitmap Logo = new Bitmap(CompanyLogo.Image);
2) select the background color (here the left top pixel) and make it transparent
3) Finally convert it back to the image
CompanyLogo.Image = (Image) Logo;