Mac’s Automator should have the functionality for you to knock this out.

In short, you basically want to:
– launch the Automator on your Mac,
– create a new Workflow,
– add Action that allows you to specify input folder and filetype,
– add Action where it automatically picks file at random,
– show you a Preview of the image, to be sure you like it.

Or you could possibly write a shell script to do the same thing even faster. Depends on your proclivities.

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This guy did something similar, but I haven’t pulled apart his DMGs to look at the detail. Hence, cannot recommend it. But maybe it’ll help.

Would a simple random number generator do?

On OS X you could make a text file called random.rb and put in it:

puts 1 + rand(30)

That one line will give you a mostly random number between 1 and 30 (including 1 and 30). And if you save the file on your Desktop you can run it from the Terminal with:
Macintosh:~ yourusername$ ruby /Users/yourusername/Desktop/random.rb

It isn’t what you want but it will help with choosing a number by random. Or you could buy a few pair of dice.

I like johnpowell’s dice idea thumbs up