Foreign image sources
Image Serving supports access to source images on foreign HTTP and FTP servers.
To specify a foreign URL for a src= or a mask= command; simply delimit the entire embedded URL with curly braces:
Full absolute URLs (if attribute::AllowDirectUrls is set) and URLs relative to attribute::RootUrl are permitted. An error occurs if an absolute URL is embedded and attribute:: AllowDirectUrls is 0, or if a relative URL is specified and attribute::RootUrl is empty.
Foreign images are cached by the server according to the caching headers included with the HTTP response.