You need to add a WM_RBUTTONUP case to display the current bitmap by using the BitBlt function. Add the following statements to your window function:
case WM_RBUTTONUP: /* message: right mouse button released */
hDC = GetDC(hWnd);
BitBlt(hDC, LOWORD(lParam), HIWORD(lParam),
Bitmap.bmWidth, Bitmap.bmHeight,
hMemoryDC, 0, 0, SRCCOPY);
ReleaseDC(hWnd, hDC);
break;