Adding ‘rel’ attribute to <a> tag in iimage editor

Hello everyone! I just wanted to add the rel=’lightbox’ in every thumbnail that is generated from the iimage editor, so i can use the lightbox image viewer in my blog. I edited the iimage-browser.php adding the rel=’lightbox’ in the appropriate values, but when i click ‘insert into post’ button this attribute is lost! Additionaly if i edit the html of the post and put it manually from there, it is not appearing as well! I figured that WordPress has a sort of validation system that removes every other attribute from the ‘a’ tag and leaves only the ‘href’ and ‘title’ …How can i edit this thing in order to accept the rel attribute?

Thanks in advance!