drop image

Assuming that i have an image of a person, then i load this image into a panel control on form. The size of the panel and the image can be difference but the image must fit in the panel by its size. After this i use mouse event to draw an rectangle that cover the face of the person on the image (the face is inside the rectangle). Now i want to create a new bitmap that just only contain the face inside the rectangle. How can i do (drop image like photoshop)??? The problem is that the image and the panel is difference from their sizes. So when i determine to the face it can be difference with the real size of the image????? Would somebody mind help me some ideas????? Thank you.