Where can I change the open image in new window code?

I’m quite familiar with the built-in option in WP to open images in a new window in posts, but this produces the target=”_blank” markup which is not valid in XHTML transitional doctypes. I actually have a javascript workaround for this:

onclick=”window.open(this.href); return false;”

Rather than manually edit every post, it would be nice to find the script in WordPress that generates the target=”_blank” code and replace it with my Javascript code. Can anyone who is familiar with the WP scripts tell me if this is possible please.

Thanks