One of the great things about collecting cards is that the only limits to what "belongs" in a collection is limited only in the mind of the collector. Simply put, a collection is only as big or as small a collector desires it to be.
Some collectors consider baseball-themed postcards as a subset of baseball cards. Several teams have issued their own sets of player cards over the years, many featuring single players. Many players have used their own photographs for autographs (I featured a Yogi Berra postcard here), and postcards have even been used for hobby-related subjects (Here's a reprinted photo, and here's a show announcement). Even Exhibit cards were sometimes used as postcards.
While going through my images, I found a few that are baseball-related postcards:
Postcards appeared shortly after the postage stamp in the 1840s and printed pictures showed up in the 1870s. In the U.S., private companies weren't permitted to offer their own postcards until 1898, but they caught on pretty quickly after that.
At first, the U.S. Postal Service held the right to the word "postcard," so they were called "souvenir cards" and required a notation that they were private mailings until 1901. After that, they were divided on the postage side (which allowed the mailing address to take up one side) in 1907.
While the previous images were only related to baseball in a generic way, I like this one because it's more specific to the sport. Plus, it's humorous rather than sentimental: