Import / Export should be able to handle this for you:

http://sdk.episerver.com/library/cms5/Developers%20Guide/Copy%20functionality.htm

Look in the SDK for the DataExporter and its property "Exclude files". Then it should copy the files to the other site as well.

Or just set up a mirroring job, hook into the PageExporting event and only add the page if the checkbox property is set to true.

http://world.episerver.com/Modules/Forum/Pages/thread.aspx?id=49189

The simplest way would of course be to set up a mirroring channel and let the editors/administrators do the work for you ;) 

Toni: Thank you for your reply, as always.

Regarding DataExporter. Hm, ok, so the idea is to somehow export the files (images), copy them, from one global files to another global files, and this should be done if the check box is checked. This is done by one export code with c# in a .cs file for Site A:s project. And one import code with c# in a .cs file for Site B:s project. And this works even though the sites are different projects?

Correct so far? If so, lets continue. In what .cs files? Export code in 'news.cs' in Site A:s project? Import code in what .cs file in Site B:s project (the .cs file to the apsx file where the news are displayed)?

Regarding Mirroring job. Hm, will that solution be in "conflict" with the rss flow that the prevoius developer did? I sort of just want to complement his code with this functionality so the images displayes properly as well. I would prefer if I didnt have to re code everything for this.

Another idea from a friend (he looked at it very hastly). Here is a code snippet from a code behind file:

XmlNode itemBody = doc.CreateElement("mainbody");
itemBody.InnerText = page["Content"] as string;
// Here, some sort of search and replace
item.AppendChild(itemBody);

Where I have commented there should be some sort of search and replace. Any idea what to write?

The source code (as I mention in my first post) says this on site B:
 /Global/_bilder/Nyheter/2011/theImage.jpg

I want to change this to:
http://www.siteA.se/Global/_bilder/Nyheter/2011/theImage.jpg

Would this work (in the comment place above)?
itemBody.InnerText.Replace("/global/","http://www.SiteA.se/global/");

I did some tests on the rss template's (aspx file) .cs file on Site A. I then looked at the result on a page in Site A with the page type (rss template).

First I tried this, nothing happened to the img src.

XmlNode itemBody = doc.CreateElement("mainbody");
                itemBody.InnerText = page["Content"] as string;
                itemBody.InnerText.Replace("/global/", "http://www.siteA.se/global/");
                item.AppendChild(itemBody);

Then I tried this but got an compile error.

XmlNode itemBody = doc.CreateElement("mainbody");
                itemBody.InnerText = page["Content"] as string;
                string newstring = itemBody.InnerText.Replace("/global/", "http://www.siteA.se/global/");
                item.AppendChild(newstring);

2 errors:
# The best overloaded method match for 'System.Xml.XmlNode.AppendChild(System.Xml.XmlNode)' has some invalid arguments
# Argument '1': cannot convert from 'string' to 'System.Xml.XmlNode'

Any ideas? 

This did the trick, from what I can tell from the source code. I have to test it live though, but pretty sure it works.

XmlNode itemBody = doc.CreateElement("mainbody");                
itemBody.InnerText = (page["Content"] as string).Replace("/Global/", "http://www.SiteA.se/Global/");
item.AppendChild(itemBody);