What is the difference between Images & Galleries?

Images are found under Media. Single images can be uploaded under Images to be used individually within a page of content.

 Galleries are found under Modules. Galleries are modules with multiple images attached i.e banners, gallery page

When you first choose your theme there will be placeholder images showing on the website, they are not viewable in the CMS.  Once you add new images the placeholder images will be replaced by the images you added.