Cosimo is the acronym for “Change Of Scene Image Many Often”. Static background images are boring very quickly. Cosimo wants to avoid this.
Users of NextGEN gallery can prepare a gallery of wallpapers and use them on Cosimo settings.
A string in the glob-style format, for example, *69*Summer, Spring?-Break*, etc. can also select images from the media library.
The change interval may following a number of page views or time (minutes, hours, days, weeks, months, years done).
The background image is used on the site about inline CSS, as in this example:

body {background-image:url(http://…../wp-content/uploads/bg-superduper.jpg) !important;}
Cache tools such as WP Super Cache can cause additional delays, especially in the range of hours. If necessary, the plugin could be expanded to also change the Header image. User feedback is welcome.