I would like to use a custom reset icon, and thought the way to do this is to add the link as a paramater, like this:

printSearchForm(NULL, '', NULL, '', NULL, NULL, 'http://www.jacktummers.nl/wp-content/themes/wp-theme/images/reset.png');

But this code gives me an error. What is the proper way to do this?

It helps to look at the documentation:
* @param string $reseticonsource optional theme based icon for reset search icon
So it must be within your current Zenphoto theme naturally.

Best count your parmeters. You passed your icon source as the object list within which to search.

My mistake btw, the icon indeed does not need to be in the Zenphoto theme. Didn't look at the source code..