This issue relates to a thread at this forum:

Mirroring is successful but no images are transferred
http://world.episerver.com/Modules/Forum/Pages/thread.aspx?id=49356

I've added an initializer and an event handler. I'm able to log the events and I notice that the e.PropertySource.Value contains permanent links. However, no images end up at the target site. Only images by built-in properties are being exported.

I've tried to use this code: e.PropertySource.Value = new EPiServer.SpecializedProperties.PropertyXhtmlString().ReplaceHtmlLinks(e.PropertySource.Value, new LinkReplacer(transferContext).ExportLinksAndAddFiles);

I've checked the ExportLinksAndFiles in the SDK, and it says the following: "A helper function to add files referred in the XHTML to a export package". I suppose this method is ment to be used by properties of PropertyXhtmlString but I can't see other ways to mark files for export.