The <a:listItems> tag is just a wrapper around a list of UIListItem objects (themselves JSF components). So there are 2 ways you can supply the images to choose from. You can either have individual <a:listItem> tags or you can have a single <a:listItems> tag and retrieve the images from a bean.
So, for your scenario you need to implement exactly what the CreateSpaceWizard does, have a look at /jsp/spaces/create-space-wizard/details.jsp, this has an example of using <a:listItems>. You'll notice the value binding expression is looking for a method called getIcons() on the bean representing the wizard. In this case it's CreateSpaceWizard. Have a look at the getIcons() method for an example of how to build the list required.
Hope that helps.