Drawing on bitmaps

Question

Let's say i create an bitmap with the size of W=256, H=256 on "Form" which have the size of 800,600. And i want to place it where ever i want, perhaps in the middle of the "Form", and draw on it with the mouse_move event. And i use the bitmap.setpixel function. I can do it, but only when i draw in the "Form" coordinates between 0,0,255,255, outside that, it's throwing an exception(crashes). I understand why it behave like that, but how can i move it?. I doing an simple drawing application, and i want to draw on the middle of the screen, on the bitmap, HoW????.