I think some screenshots of the broken images and relevant sections of the HTML source code would help folks that would like to assist with troubleshooting.
I believe most Discourse image uploads are for the relative URL path, so I wouldn't think that changing the hostname should matter.
Speaking of hostnames, do you have any hostname assigned to your Discourse instance? It sounds like you might be just trying to access based on IP addresses only and I'm not sure if Discourse is designed to work that way.
Did you upload the image file itself to Discourse or did you just point the to an image file located somewhere else on the web (or your local intranet)?
You may also want to try Rebuilding the HTML for a post to see if that helps with the issue, that option is usually under the wrench menu on a post.