Sorry, I am not quite sure what you mean "when someones uses the default search of wordpress" ... You want Wordpress to "search images"? Based on what data? This data would need to be stored in some database. Besides, after the search, are you just gonna display all related images inside the wordpress page? I don't get it ...
Sorry, maybe I was a bit too short.
Wordpress has a search field by default in the header area. If you write something in that and hit enter, you will get all items from WP's database, more precisely from the POSTS table (usually "wp_posts").
And you are right, there are two things to solve here:
1) things have to be stored in the POSTS database table, to be searchable by the WP script. To do this, we need the WP plugin to open the XML files of IV and create a special POST in the POSTS DB table according to the FILE items in the XML file. And we also need the WP plugin, to go back occasionally and check if there were changes in any of the XML files. (Best, if it is only done when something has changed in the XML file.)
2) once the search finds something, in the POSTS database table, the WP search listing has to show the file with a link in IV. For this, the plugin has to overwrite the links generated for the special posts created by the plugin in 1). This only requires the plugin to know the URL of the post where the IV is. So it will put the post url and the file together with a #, and we have a working link.
I hope, I was more precise now, and you get my concept.