You have to create each part as a separate image.
1) Load the whole image, and (say) you want it in four equal pieces.
2) Create four new images large enough to hold each partial image.
3) Get the Graphics context for each sub image in turn (using Graphics.FromImage
]) and use the Graphics.DrawImage overload
] which allows you to specify the location in the destination, and the area of the original that you wish to draw.
4) You can then dispose of your Graphics objects, and save your images.