SendMessage and image question

I''m using C# and imported the SendMessage function, it works cuz I use it in other progs so theres no prob there. I want to use BM_SETIMAGE to set an image for a hWnd, the hWnd is also correct cuz I have used a WM_CLOSE on it to test it. I tryed:

int BM_SETIMAGE = 0x00F7;

[DllImport("user32.dll", CharSet=CharSet.Auto)]
public static extern IntPtr SendMessage(IntPtr hWnd, int msg, int wParam, int lParam);

SendMessage(hWnd,BM_SETIMAGE,0,(int)myImage.GetHbitmap());
 

but it didnt work