No worries, Miles. The other thread covers a quite different aspect of the copyright laws - ie how to avoid prosecution for falling foul of them. You were concerned with how the law might be used to protect your own rights - that's a different matter.
It might be worth saying a little more, however, concerning the posting here of scans taken from old photographs that we have acquired. Copyright for photographs normally resides initially with the photographer, but if (s)he takes a picture in the normal course of employment, as would a newspaper photographer, copyright normally passes to the employer. In the case of a freelance commissioned and paid to take a photo or set of photos, again the copyright would normally reside with the customer unless a different arrangement was negotiated as part of the contract.
Let's take the case of a photo of Olympic taken by an amateur in 1930 and pasted into a family album. We'll call the man with the camera Fred Dobbs. When Fred took that picture he had no thought that it might some day be of value and might even be worthy of publication and he probably had never heard of the concept of 'intellectual property', but nevertheless the image he created was still protected by the law of copyright. Let's say Fred died in 1950, and all his property was inherited by his only daughter Mabel. Mabel's inheritance included Fred's 'intellectual property' - like the copyright on all the photos in his album. And that copyright would not expire for another 70 years - ie in the year 2020. The 70 year ruling is now current in the US as well as the European Union. In some other countries, like Australia and Canada, the term is 50 years. So Fred's picture of Olympic could now be published in either of those countries with no fear of copyright infringement.
Mabel of course had no idea that she had inherited anything other than physical objects, including the album and its contents, and since she had no interest in her father's collection of photos of old ships she later sold it to a junk shop, gave it away to Oxfam or whatever. It changed hands several times, and eventually one of us buys that album and gets excited about the pictures it contains, and of course we immediately want to share those images with our friends on ET so we scan them and post them here.
Now, theoretically when we do that we are in breach of copyright, because somewhere out there is Mabel, or a descendant who still owns the copyright to those images which will be valid for another 16 years in the UK or US. But the chances that Mabel, or Mabel's heir, remembers the photo, knows that they have rights connected with it, and is aware that it was posted here are, to put it mildly, slim. The use of such images is what publishers refer to as a 'calculated risk', and a very small risk at that. The normal procedure on web pages is to make a statement that you have not been able to trace any copyright holder, but if anybody comes forward with evidence that they have such rights and wish to have the image removed, you will do so.
In the case of posting a photograph that was taken professionally for publication or for an official archive, the chances of being in breach of copyright and coming to the notice of the copyright holder are much higher. But even in such cases, if you have made no money from the use of the image the worst that is likely to happen is that you will receive a 'cease and desist' order - ie remove the image or further action will be taken by the copyright holder.
Members wishing to post images or text on American-based websites should note that in the USA anything first published in that country in 1912 (anything before 1923, in fact) is now in the 'public domain' and can be reproduced freely within the US.
Hope that helps a little. I am not a legal professional, so if anyone with expertise in this area can add to this summary or correct any errors of interpretation that would be appreciated.