CardCow continues to be one of my favorite websites. It’s an online postcard store, but they have such a huge collection that it’s a really interesting site to search or browse even if you’re not planning to buy any postcards. They keep all the images online even after the card is sold, and they scan both sides which is often helpful when you’re trying to date an image. (And some of those messages are pretty interesting as social history!)
There’s also a nice feature that makes it easy to add these images to your website. Just click on Add This Card to Your Web Page and you can get the right code for to copy and paste to add the small, medium or large version to your site. The image displays a subtle watermark and will be linked back to the CardCow website.
They have lots of vintage holiday images that would work well on library booklists, blog posts and other websites, like this Thanksgiving postcard: