Above is the tutorial. It is all plain PHP stuff you should know the basics of if you try to modify a selfhosted tool like Zenphoto.
So as with the download list: Place the function above with the parameters set as wished on the theme where you wish to use it. A good idea is to read the theming tutorial.
Comments
Please take a look at the documentation yourself. We have tried to name all functions "speaking".
SOme common links:
http://www.zenphoto.org/news/functions-documentation
And how to read the doc:
http://www.zenphoto.org/news/how-to-read-the-zenphoto-functions-guide
So as with the download list: Place the function above with the parameters set as wished on the theme where you wish to use it. A good idea is to read the theming tutorial.