Sorry for being a complete numpty, but to post images in postings, can one use images from the gallery (if so what does the URL look like) or does one have to have a Flicker account or similar?
Yes you can. Find the URL of the actual image (usually using a browser option like "view image" or "copy image location"), and use that.
(...and see this FAQ thread in the Astrophotography section (http://forum.orpington-astronomy.org.uk/index.php?topic=1837.0).)