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That might be easier for you to read, but it's much harder for us to read. It might seem like a small thing, but it goes a long way to making it easier for us to help you.

Anyway, you're extending...

Okay, and where is the code for displaying a background image in MainWindow? (which should be mainWindow according to the standard naming conventions, by the way)

You have 3 JFrames here. Which JFrame are you trying to add the image to?