When in French they are called carte postale the same was as in English. I also noticed that on some old European postcards they wrote "postcard" on several languages.
On the other hand to distinguish a postcard form just a a picture on a thick card-stock you may want to use the reverse side to put some information about the image or the company. Traditionally postcards also have some lines to make writing address easier.
Here are some examples of the reverse sides of the postcards.