In the Gallery, folders are referred to as albums.  On the main Gallery page, there is an option on the top left-hand side to 'Add Album'.  Once you have selected an album, in the same top left-hand area, there is an option to 'Add Photo'-use this to upload images one at a time or in multiples. There are a few articles on images and the gallery in our wiki you may also want to check our for more information.

Folders are used in the documents area of your community-accessible from your site manager.  Here the funcitionality is very similar, click 'Add Folder' to create a folder and 'Add File' to upload a file. This wiki article gives more inforamtion on folders.