Combining several Bitmaps

I am taking several bitmaps and drawing them onto a picturebox with a low opacity so that all of the bitmaps can be seen together as one picture.
My problem is that any time I need to re-draw this "final combined image", I have to put all of the pictures on top of each other again, which is slow when you're dealing with 100+ bitmaps.
How might I be able to draw each bitmap (with opacity) onto a new "final bitmap" rather than onto the picture box so that I can just paste in the final result quickly?

Figured it out.
Just make a graphics from the final bitmap and draw each individual bitmap onto it using the same color matrix and it still works.

Glad you solved your problem. If you're dealing with blending images with transparency you may also be interested in alpha blending; link: http://en.wikipedia.org/wiki/Alpha_compositing