Longer answer: A postcard or other work needn't have a copyright notice to be protected by copyright. The only postcards you can safely copy are those that you know are in the public domain. Project Gutenberg has some useful guidelines:
EFV is right; virtually everything is copyrighted. You probably wouldn't get into trouble for using one postcard, but since you're speaking in the plural it sounds like you want to use a bunch of them to pad out your site's content. THAT is not a good idea.