Each "it" catalog can have a background image, selecting other images in the catalog will display them over the top of the image in a non-destructive way. To set up and use this feature do the following:
2. Now view your foreground image. If you image has alpha, as Mr Ray does below it will be composited on top of you background.
When you are using a background image you can also use the "wipe tool" to reveal parts of the background under the foreground.
2. left-drag the mouse in the image view to see under the foreground.
3. The direction of the wipe can toggle from vertical to horizontal by click the wipe direction button