Make a copy of the file `zp-core/zp-extensions/tiny_mce/config/zenphoto-default.js.php` as `pluins/tiny_mce/config/zenphoto-with_fullpage.js.php`
Edit that file so that the line for [i]theme_advanced_buttons2[/i] reads `theme_advanced_buttons2 : "undo,redo,|,link,unlink,anchor,image,cleanup,help,code,fullscreen",`
Then go to the tiny-mce plugin options and select `zenphoto-with_fullpage.php` as your "Text editor configuration - Zenphoto" selection.
This will put the full screen button in the button set and allow you to edit full screen.