Shadowmap,
Sottwell is right in her diagnosis -- if you simply change the name of the machine from "localhost" to the actual name of the machine, everything will work. This is not a modx problem at all.
Localhost always refers to the machine you’re on -- therefore when you access your site from another machine, and it sees "localhost" in the source, it look to *itself* to get those images.
Lots more info here:
http://en.wikipedia.org/wiki/Localhost
Anyway, this is very much a network config issue, not a modx problem.
