What you probably want is what’s called a “binding.” Simply put, in your widget blueprint, create three Slate Brush variables. Two of them will store the two images you want to switch between (call them BrushA and BrushB, for example), and the third will basically be where you store the currently loaded image (call it BackgroundBGBrush, for example). Set the default values to whichever image you want to be in each, and in BackgroundBGBrush, make sure to set its default value to whatever you want to show up by default. Compile, then go to the Design View and click on your background image. Under “Brush” you’ll see a button that says “bind” click the drop-down and look for the variable you want to use to store the currently loaded image.
From there, it’s as simple as going to your “Play” button’s on-click event, and setting “BackgroundBGBrush” to “ImageA” or “ImageB”. Check out the [UMG Inventory UI Tutorial by Epic][2] to learn more about how UMG works.