<div class="hentry">...</div> to determine to which post/page an image belongs. A lot of WP themes live by that convention but both your themes do not place post/page content in a div with that class.
The next release of Easy FancyBox will have an option to make it compatible with your theme. In fact, you can try the development version right now: http://downloads.wordpress.org/plugin/easy-fancybox.zip It has this feature already along with a new "Spotlight" effect when opening images...
Paula, if you decide to try it, set the new option "Group selector(s)" to "div.postarea". Max Engel, in your case it's "div.post" that should do the trick. Then you can try different variations of the two new options above it, controlling the FancyBox enabling and gallery mode :)