The Joomla! content management system lets you create webpages of various types using extensions. There are 5 basic types of extensions: components, modules, templates, languages, and plugins. Your website includes the extensions you need to create a basic website in English, but thousands of additional extensions of all types are available. The Joomla! Extensions Directory is the largest directory of Joomla extensions.
Modules are small blocks of content that can be displayed in positions on a web page. The menus on this site are displayed in modules. The core of Joomla! includes 24 separate modules ranging from login to search to random images. Each module has a name that starts mod_ but when it displays it has a title. In the descriptions in this section, the titles are the same as the names.
Templates give your site its look and feel. They determine layout, colours, typefaces, graphics and other aspects of design that make your site unique. Your installation of Joomla comes prepackaged with three front end templates and two backend templates. Help
Plugins are small task oriented extensions that enhance the Joomla! framework. Some are associated with particular extensions and others, such as editors, are used across all of Joomla. Most beginning users do not need to change any of the plugins that install with Joomla. Help
These are my photos from parks I have visited (I didn't take them, they are all from Wikimedia Commons).
This shows you how to make a simple image gallery using articles in com_content.
In each article put a thumbnail image before a "readmore" and the full size image after it. Set the article to Show Intro Text: Hide.