It's not clear to me if this is a one-time fix or something that will be applied to incoming HTML data on an ongoing basis.

If it's one time, s{<img src="(?!/)(?!http:)}{<img src="http://domain.com/}g sounds like what you want. (Suppress replacement for both src="http:... and src="/...)

Do you really want the full http://domain.com/ there? Or would an absolute path beginning / be good enough? The latter might save you headaches if you start using multiple domains or secure pages (where some browsers will complain if there are non-secure references on the page).