Hold the left mouse button down as you drag the Slider control (labelled "Set Fade Level") above the image (or click on the arrow buttons) to fade between the two images. To zoom, click on the "Zoom" button, and then click the "hand" cursor on the image where you want to center the zoom. To toggle between the 2 images, click in the middle of the grey Slider control area, beneath the word "Fade" in "Set Fade Level".