Before going on to answer your question, I should just make it clear that the statement "UK copyright on images like old postcards lapses after 50 years" is a bit of a generalisation. Under UK law, virtually all photographs made before 1945 are now free of copyright, but a postcard which featured some artwork, say like a McGill cartoon, would be subject to the longer copyright term of the artist's lifetime plus 50 years if the artist's death occurred before 1945, or their lifetime plus 70 years if the death occurred after 1944.
But with regard to your question, yes, the copyright term for something created or first published in France will be subject to a different term of protection. Indeed the whole approach to copyright is somewhat different but that need not concern us here. Prior to the Law of 1 July 1992, the term of copyright protection in France was the lifetime of the author plus 50 years from the end of the year they died. After the 1992 Law came into force, it was the lifetime plus 70 years. The extension of 20 years applied to any work which was still in copyright on 1 July 1992. But there are number of important modifications to this basic rule, because France recognised the effects of the First and Second World Wars on the ability of copyright owners to enjoy their rights. The best way to explain how this works is to quote the French law:
The rights granted by the law of 14 July 1866 [ie the lifetime plus 50 years] on the rights of the heirs and successors of the authors to the heirs and other successors of authors, composers and artists shall be extended by a period equal to that which elapsed between the 2 August 1914 and the end of the year following the date of the signing of the peace treaty for all works published before that date and not in the public domain February 3, 1919.
The rights granted by the law of 14 July 1866 and cited in Article L. 123-8 to the heirs and successors of authors, composers or artists are extended for a period equal to that which elapsed between September 3, 1939 and 1 January 1948 for all works published before that date and not in the public domain as of August 13, 1941.
The rights mentioned in the previous article are further extended, for a period of thirty years when the author, composer or artist died for France, as is clear from the death certificate.
If the death certificate was neither drawn up nor registered in France, a decree of the Minister of Culture may extend to other heirs or successors of the deceased the benefit of the additional extension of thirty years; this decree, issued after the authorities referred to in Article 1 of Ordinance No 45-2717 of 2 November 1945, will intervene only in cases where the words "died for France" would have appeared on the death certificate if he had been trained in France.
As you can see you may have to do a bit maths based on when the postcards were published and also if the photographer was deemed to have died in the service of France in either the First or Second World War.
If the photographer is anonymous the folllowing rule applies:
For a pseudonymous, anonymous or collective work, the duration of the exclusive right is seventy years after January 1 of the calendar year following that in which the work was published. The publication date is determined by any mode of proof of common law, including the legal deposit.
If a pseudonymous, anonymous or collective work is published in installments, the period runs from 1 January of the calendar year following the date on which each item was published.
When the author or authors of anonymous or pseudonymous works made themselves known, the duration of the exclusive right is that provided by Articles L. 123-1 and L. 123-2.
The provisions of the first and second paragraph shall apply only pseudonymous, anonymous or collective works published during the seventy years following the year of their creation.
However, when a pseudonymous, anonymous or collective work is published after the expiry of the period mentioned in the preceding paragraph, its owner by succession or other securities, which carries out publication enjoys an exclusive right of twenty-five years from January 1 of the calendar year following the year of publication.
However, bear in mind that 'anonymous' is not the same as 'currently unknown'. If it is possible to discover the name of the photographer by dilligent research, then Article L 123-3 will not apply. Another way of looking at this is to ask yourself if the identity of the photographer would have been known to the publisher of the postcard. Since this seems likely, Article L 123-3 won't apply in most cases, which leaves something of a problem if you need to find the date of death of an unknown person!