When you add images to AutoCollage from Windows Live Photo Gallery, the AutoCollage Image Browser pane title changes from 'Image Browser' to 'Live Images [n]', where [n] is the number of images added to AutoCollage.
You can add several images to AutoCollage at once. If you want to add more images to AutoCollage in 'Live Images' mode, select those images and use the Add To AutoCollage... item in Windows Live Photo Gallery. This will add any newly selected images to the AutoCollage image preview pane.
Note that you cannot use this interface to remove images from AutoCollage, only to add them.
See also: Browsing Images, The Image Browser Pane