You are using a very, very old jQuery version (1.4.4). Responsive Lightbox is trying to use functions that are are not available in that version. More: you are also using an old (3.1) version of WordPress.
The plugin does not generate thumbnails – it generally does nothing to your theme. If configurated correctly, it just adds lightbox effect to your image links (includes a JS script and addts rel attribute to your images).
Hard to say what in your theme does such thing, but believe me the plugin can’t do this – there is no a single line of for that. It just adds rel attribute to your image links. Maybe your theme changes the image links output if it has a rel attribute applied. Did you analyse how your theme handles images?