Hi Jules

Do you have the actual photograph? If you can scan it again, as Irene suggested, at 300 dpi and repost, you may get better results (not that Irene's restore is not good, but she and others will have more information to work with!).
Perhaps you could select just the portion of the photo with the subjects in it, cropping out the background, to make a smaller file size.
Also, any information about the card would be useful - is there anything on the back? how big is it?
At first glance I thought the photo looked quite early, maybe 1870s, but there isn't much to go by

Cheers
Prue