Mask includes a number of built in wall images, but you may have your own, or ones from your clients, that you want to use.
You can setup your own new wall image using a background image that you have.
In Mask, create a new wall image by right-clicking on a String or Image and selecting Wall Image/New from the menu.
On the Select Image screen click the Browse button and browse for your background wall image. Once you have selected an image click it will be added into the list.
Click OK and your Wall will be loaded, but it it isn't quite ready yet.You still have to configure the area of the wall that you can place images onto.
To do this right-click on the Wall Image and select Wall Image/Configure Wall Settings.
This will change to full screen mode and display a selection rectangle that you can modify to define the wall area. There will also be a small settings window that allows you to set the height of the wall so that your images will be scaled correctly.
Now drag each of the corners of the selection to where they should be.
Once the selection area is correct enter the height of the wall. If in doubt use 8 feet.
Once everything is setup correctly you can save the settings, either as a Client Preset which will only be available to this client, or as a System Preset which will be available on all clients, just as the built in ones are.
To exit Wall Configuration mode click the X button on the small settings window.
You can now add images to the wall image.