Hi there sammymocha!
Thanks for an interesting research question.
The short answer is yes, you probably can reproduce them at will.
Before I elaborate, may I just take a moment to warn you that this
advice should not be substituted for that of a legally qualified
professional, which I am not.
Assuming that your postcards were created before 1923, they are now in
the public domain, and may be copied freely.
The Library of Congress makes this clear, stating that "Works
copyrighted before 1923 are now in the public domain". This (and more
information) can be found in their slickly titled "Copyright and Other
Restrictions Which Apply to Publication and Other Forms of
Distribution of Images: Sources for Information":
If you think the work was created after that (between 1923 and 1964,
when new legislation came into force) then you have a rather tougher
situation on your hands. You must take steps to trace the original
copyright holder. This is a long, hard, and not particularly pleasant
road to journey on, and the difficulty probably outweighs the reward!
The information is available here:
I hope this has answered your query. If anything is unclear, or you
wish me to explain something in more detail, then please do not
hesitate to request clarification before rating this answer.
"copyright expiration" images
"copyright expiration" photographs
"copyright expiration" pictorial