Some of my users do extensively searches in my blog, using the wordpress search. Is there a (simple) way to include the zp search in the wp search.php? I think about displaying both search results (zp/wp) with a horizontal or vertical spacing ...

I don't think there is any "simple" way. You would have to setup the zp search envronment on you wp page. Then you can use the next_album() and next_image() functions. To setup the environment you need to have the search parameters 'posted' and call the zp_load_search() function.