C# :: Save Image In Layers And Then Load Layers Separately

I am programming an application in c# that allows the user to edit images. The user can use a Pen to write on the image, and the user can also use a highlighter (a big semi-transparent Pen) to highlight what they've written. I need the user to be able to erase only the highlighting without erasing the pen drawings.

My solution was to have the user draw on one graphics window when the pen is selected and draw on the other graphics window when the highlighter is selected. I will then enable the eraser on either graphics window depending on what the user wants to erase.

My program will let the user save the image and then load it into the program again for later editing. When I save the image, I have to compress the layers (different graphics windows) into one bitmap. When I load the bitmap back into my application, I lose my layers and cannot erase just the highlighting without also erasing the pen as well.

How can I save an image file with layers, and then recover the layers when my program starts up again?

I am not opposed to saving two images - one with the pen graphics window and the other with the highlighting. If I do that, how can I merge the two images into one layer when I load the images back into my program?