Manually add lightbox to wordpress (without a plugin)

hi there,

can you guys tell me where i can put a rel=”lightbox” attribut in my anywhere in my wordpress code to enable my lightbox. i download a normal jquery lightbox plugin (not a wp-plugin) that i want to use! however i need to add the rel-attribut to images in the_conent!

have you guys any ideas?

thank you very much!