The following applies to SharePoint 2013 and SharePoint Online / Office 365.
Theme Background Images
When designing a theme for your site you can use a background image for the page. Did you know that SharePoint 2013 includes more than fifty 1024 x 768 sample images? Ten of the sample themes use the first ten of these images. These ten images and forty more can be found at the following URLs:
Replace the “image_bg002.jpg” with the names from the samples below.
Background images used with the sample themes:
Additional Background Images Available:
And a few more interesting images from that folder: