The compare tool is for comparing two different images. To enable, click the box above. You should see a black rectangle appear over the current image. Then select a date and time from the calendar or use the playback controls to change the background image. To select a new image, just toggle the the above check box on the image you want to grab.
The interval tool is for skipping a over certain amounts of time to speed up the time lapse. Choose any interval you would like to skip over that many images.