Mirroring will automatically fetch resource files from Vpps (e.g. an image) that are referenced by e.g. a PropertyXHtmlString or PropertyUrl (includign PropertyImageUrl and PropertyDocumentUrl).

Since it does not work for you I guess you have a custom property?

One option is to change the property to one of the built in, then you will get the mirroring support for free. If you want to use a custom property you need to write a transfer handler for your property (see e.g. http://sdk.episerver.com/library/cms5/Developers%20Guide/Copy%20functionality.htm#custom_properties). If your property stores data as a  url you can probably call generic method on PropertyUrlTransform to handle the transform like:

 public class CustomPropertylTransform
    {
        public static void ImportEventHandler(object sender, TransformPropertyEventArgs e)
		{
            PropertyUrlTransform.ImportEventHandler<CustomProperty>(sender, e);
        }

        public static void ExportEventHandler(object sender, TransformPropertyEventArgs e)
        {
            PropertyUrlTransform.ExportEventHandler<CustomProperty>(sender, e);
        }
   }

 If your property is inheriting PropertyXhtmlString you can look at implementation of EPiServer.Core.Transfer.PropertyXhtmlTransform.ExportEventHandler (through reflector) for an example.

In that implementation the call to ReplaceHtmlLinks is the part that parses through the string and find all linked resources and adds them to export package.

e.PropertySource.Value = new EPiServer.SpecializedProperties.PropertyXhtmlString().ReplaceHtmlLinks(e.PropertySource.Value, new LinkReplacer(transferContext).ExportLinksAndAddFiles);