Use the Image Browser pane to select which folder to view in the Image Preview pane. You can also use this pane to select a different Image Browser root folder.
The title bar of this pane is usually charcoal-colored. However, when images are being loaded from the folder, it becomes a progress bar, slowly turning green. When 1/4 of the bar is green, 1/4 of the images in the selected folder have been loaded. In the example above, just under a quarter of the  images in the Monuments folder have been loaded.
To the right of the title bar is a small button with ellipsis. Select this button to change the root Image Browser folder (the Pictures folder in this example). Only the root Image Browser folder - or the folders beneath it - can be used for making AutoCollages.
The body of the Image Browser pane contains a folder hierarchy. Sub-folders are displayed indented beneath the folder they belong to. A folder with a small triangle icon next to it - such as the faint triangle next to the Pictures folder - has sub-folders. If sub-folders aren't displayed, clicking on the triangle next to the parent folder will show them.
Some folder names have a number in square braces next to them. This is the number of recognized images in that folder. If the number is [300+], there are at least 300 images in the folder, and AutoCollage will only try and load the first 300 if that folder is selected.
AutoCollage can be used to create collages from pictures selected via Windows Live Photo Gallery. When images in the Image Preview pane are from Windows Live Photo Gallery rather than an image browser folder, the title of the Image Browser pane is replaced with 'Live Images' and an image count, as shown below.
To return to folder browsing, simply select the back arrow button next to the title.
See also: AutoCollage Application Panes, Browsing Images, Using AutoCollage With Windows Live Photo Gallery