The bit you cite is of course aimed at the old post form years(?) ago… I don't know anything about your install naturally.
The function is name, well, randomImage… But you don't edit "functions", you edit your theme files. Sorry, there is no simpler answer. To learn the basics about Zenphoto theming read:
http://www.zenphoto.org/news/theming-tutorial
The Efferscence theme is a bit more complicated than described in the tutorial but it would be the `indexpage.php` file of the theme. See the "daily image" options at the beginning.