The gallery container
Add block - Utilities - Media - Gallery
The gallery is used to hold gallery items
The gallery item will ask you to select an image file by upload (Uploads to /images) or from your existing images and to add a caption text for the image
It then creates a thumbnail in /images/gallery_thumbnails for both uploaded and existing images
The thumbnail has _thumb and a id code appended to the name
The caption text is used in the title attribute of the link and the alt attribute of the image
It then creates a image link to the full image using the thumbnail