Hi there, I need to edit the default template to make the "clicked" image appearing over the thumbnails and not in a new page. What php lines do I have to edit?

This is my situation-page: http://gleenk.com/test/prove/
and I need to make image I click on appearing over the thumbnails.

You have to setup your theme to use a "box" javascript. Until version 1.2.8 Zenphoto has Thickbox on board (located in zp-core/js). With 1.2.9 that will be replaced with Colorbox. Info how to setup these scripts on their sites. Of course you should read our theming tutorial as well. (btw, your site does not use the default theme but a third party theme).

I used the default theme but i removed everything :) I haven't understood well how do i have to do, i've seen that "flickrish theme" is similar to what i need but it displays only 2 thumbnails preview in the image-display page. Aren't there few php lines to make this in the default theme?

You can do nearly everything with Zenphoto themes. Again, please read the theming tutorial.