Realize that the hwnd is the window handle of the picwin container not the picwin contents. There's another hwnd inside where mIRC draws. You should use a tool like Spy++ to find out what the window structure looks like and you can FindWindow to get the hwnd from the parent hwnd that mIRC gives you.
After you get the real hwnd you need to hook into the window (SetWindowsHook) and handle the WM_PAINT (not PRINT) message.
