If you want to know how the image gallery works, extend it, or simply are interested, it is best to read through all developer documentation and the API on this Image Gallery homepage.
This page describes how to get information out of the API code-wise. If you want to know other handling operations (e.g. creating or deleting new Content Folders) look at the bean view classes spcp7.imagegallery.view.managedbean.EditPropertiesBean.java and spcp7.imagegallery.view.managedbean.GalleryViewBean.java within the spcp7-imagegallery project and their Java Doc respectively.
For this guide we look at the EditPropertiesBean.java 's retrieveCurrentBrowsedContent(ContentFace, PropertyWrapper) method which retrieves the content from a content provider. The HOWTO is inline which means directly within the EditPropertiesBean.java class. This is done in this manner to make it possible to reflect changes which are made to the code within the Howto immediately and also because Code Highlighting is not supported for the APT-Format.
If you want to see the howto online click here which opens the class with highlighting in the svn view mode. Scroll down to line 523 for the beginning of the howto.