There's no single function to do what you want. You must load an image, convert it to an appropriate format if necessary, and then use a rendering API to draw it in your window.
Btw, i have a question about child windows. Does a child window need its own windows procedure, or will it use the procedure of the parent window?
It depends what window class you specify when you call CreateWindowEx() because window procedure is a property of the window class. If you're talking about actual windows(not controls) then you could use the same procedure or not.