Mask Walls are just like Composite Images, except that your images are overlaid onto a Wall background.
Mask includes a number of default walls which you can get started with right away.
To create a new Wall Image, right click on a String or Image and select Wall Image/New.
Then select the wall that you want to use from the list.
You can now drag images onto the new wall image and work with them just like they are a composite image.
One difference with Wall Images is that there is a defined area in which you can place images.