Create 2 images to serve as doors or covers (I used Paint for this). Find 2 additional images to hide behind the doors / covers (I used Google Images). In Design view, create an image for each of the 4 images. It is a 'mouseover' event when the cursor is over an image. It is a 'mouseout' event when the cursor is not over an image. Use 'onEvent' to respond to these events and cause the hidden image to appear or disappear. The 'hideElement' and 'showElement' pages, linked to below, include examples that show how to do this.
Here are links to the App Lab documentation:
The video shows what a completed app should do.