I am quite unsure here:
Image i=some image...
Bitmap B=(Bitmap)i;
The B now points to the same object as i. I am confused...I would say that Bitmap B will point to new instance of Image that is casted to bitmap but it is obviously not the case. Then I just do not get how it works here.
Casting does not create a new object (at least, not unless new conversion operators have been defined, which is uncommon in non-numeric types, and doesn't apply in your example). It merely instructs the compiler how to "treat" an object. In the case you present, you're telling the compiler "don't worry, trust me, B is actually a Bitmap". If it turns out you've told it a fib, the runtime will catch you on it by throwing an InvalidCastException at runtime.
MSDN has some more information.
A cast is a way of explicitly informing the compiler that you intend to make the conversion and that you are aware that data loss might occur