Background Image Change With One Button

I'm trying to program a button that changes the background of my screen to randomly one of the four optional backgrounds (I'm not using colors, I'm using images), or change to one of the backgrounds in a specific order. I have tried many variations of ways to program this, but have failed to be successful.

Why not show the blocks you have tried? You are probably quite close to a solution.

Thank you so much for telling me! I have fixed the problem and the button now works as intended.