Hi Neil,
extremely beautiful couple!
Mardier could also be the name of the owner, see for example page 10 of the attached publication of honored soldiers.the publication is from 1790, this corresponds nicely with the age of the couple.
the raised hand may indicate that the owner has taken some kind of a (military) oath. It doesn't have to be a coat of arms.
best
Jasper
https://books.google.nl/books?id=bvF...0paris&f=false