A Brief Overview of Copyright Information
In general, copyright extends to 70 years after the death of the artist. In cases of an anonymous work, it extends 95 years after its initial publication or 120 years after its creation, whichever is shorter. Something from the 1950s, for example, may not be eligible for public domain status until sometime after 2045. But that date could be even further in the future depending on the lifespan of the artist and the specifics of the copyright on that item.
This is complicated by the fact that copyrights can be renewed, thus extra concern is warranted if the item is in the catalog of a still existing company. This means that items that may be quite old may not actually in the public domain yet.
The vast majority of the cards displayed in the Vintage Valentine Museum are NOT in the public domain. They are old, but not old enough. However, many of the Victorian era cards (those dating to the first decade of the 1900s and earlier) do fall in this category. This is why there are so many companies offering reproductions of cards from that era. The Old Print Factory, Merrimack Publishing, and Shackman cards can be found in museum shops, book stores and other places. My understanding (again, not a lawyer) is that you would need an original copy of any 'public domain' card on which to base your image. If you used a reproduction card to generate the image you want to mass produce, this could be problematic. Whoever produced the reproduction card might care enough to sue. Even if they do not prevail, the action of being sued can be very expensive. Expensive enough to cause real financial harm.
This gets us to an important aspect. As mentioned above, one must be especially careful in using images that a current company may have offered in their past. One example would be the Walt Disney Company. If someone were to try to use images from the cards they published in the 1930s - despite how long ago that may seem - in any sort of money making venture, Disney would almost certainly care and perhaps care deeply. That said, if you were to use an actual old card of theirs in some sort of craft product that was sold, you may or may not be relatively safe from being sued. Here is some of the nuance mentioned and is why consulting with a lawyer specializing in this area before beginning your money making project would be very important. Much can depend upon the specifics of your intended use as well as the specifics of the copyright pertaining to the particular image.
Hallmark is another example of a company still in existence, with a catalog going way back. Using one of their older cards in a commercial venture, even a small one, may create some problems. They might choose to sue. In general, if the image comes from a company that is still in existence, no matter how old the card itself may be, you are taking a risk that could cost you. Copyrights are sometimes renewed. Companies, especially large ones with deep pockets, may choose to pursue compensation for any unauthorized use of their artwork, however incidental it may seem. Large companies are more likely to renew copyrights, especially if any characters appearing are still featured in their more recent catalogs.
With this in mind, another consideration is whether or not the company in question that may no longer exist, was purchased by another company that is still in operation. Companies like American Greetings purchased many smaller companies. So even though the company whose name appears on the card may be no longer extant, there may still be a parent company that would take an interest in use of the old image.
"Fair Use" is another important concept that may be considered, depending upon exactly how the image in question is used. Satire and parody are allowed uses without the copyright holder's permission. Whether or not a particular use may fall under this guise, can be rather complicated. Here is another instance in which consulting a copyright lawyer is really the best idea. It can be a whole lot cheaper to consult a lawyer ahead of time, than to defend oneself against a suit.
So you may have gathered by now, that what is really important here is whether or not there is a company, an artist, or an artist's estate that may care about your use and therefore sue for compensation. This can include damages and court costs which have the possibility of being quite large.
Personal use, uses not generating an income, are less worrisome. Using card images generated yourself on personal crafting projects or decorating non-commercial web sites are not likely to cause any issues. Using images found on the web can be a little more complicated. While scanned images are not considered automatically copyrighted, non-scanned photographic images may well be. So it is possible for the card itself to be in the public domain, while the specific image you found of the card is most certainly not.
Remember too, that images found on the web are only there through the efforts of someone else. While you might not be in danger of being sued, using an image by re-posting it without crediting your source puts you in a somewhat shady area. And if you have the gall to actually put your name, thereby crediting yourself for someone else's time and efforts, on an image you took from the internet, you are a colossal jerk. You are why we can't have nice things. This should be obvious, yet I have seen exactly this. There is a relatively well known artist who did this to several of my images and the images of others. I'm not going to name names, but he's pretty easy to spot in a google search if you're curious. Do not be 'that' person. Don't be a freeloading jerk. Give credit to your source. Do it because you are a decent human that respects the efforts of others. Do it because it is the right thing to do and it only takes a moment. No one respects a jerk. We should all be better than that. Especially artists who would surely not like their own work being treated so cavalierly by others.