Again. You should look at the code in Darwinbots for how to do this.
Just look at the object for MDIForm1 then click the menu bar, look for the bit that allows the user to set a background image from a file then click it to see what the code says.
I can't remember exactly how it all works but I do know that Form1 has a member called BackPic into which a bitmap is loaded.
I solved all these issues way back. You will just have to check my code to see how.