Random image block from specific gallery

Hi,

I'm using Image module for image galleries and would like to create a block that displays random images from a specific gallery of my choice, rather than from any existing gallery (like the default random image block already does).

I found this php snippet at http://drupal.org/node/53987 but can't figure out how to make it work and no replies were given by its creator.

Does anybody have an idea on how I could achieve my purpose or how to configure the above php snippet correctly?

Thank you very much for your support, regards.