To show all images as a gallery, we'll use a plug-in from http://www.kubik-rubik.de/.
Navigate to http://joomla-extensions.kubik-rubik.de/sige-simple-image-gallery-extended. You can also do a web search for "Simple Image Gallery Extended", this will take you to the same page. Download the latest version; at the time of writing this file is called
plg_sige_v3-1.zip
.Navigate to Extensions | Extension Manager. Select the plugin ZIP file you downloaded and click on Upload & Install. You'll see a message when the installation is finished (Installing plugin was successful).
As this is a plugin, we'll use the Plug-in Manager to enable the extension. Navigate to Extensions | Plug-in Manager and locate the Content – Simple Image Gallery Extended plugin. Click on the red and white cross in the Status column. It turns into a check mark, indicating the plugin is now active:
The plugin has no additional settings, so you're all set. Click on Save & Close...