First, you'll need to wait until the server migrations are complete at least; until then, nothing is going to be stable enough to mess around with settings.
Second, unless you're doing a significant write-up on each of the cards, they would count as an image gallery, and that means that there will be a 1GB limit even if you have otherwise unmetered space. (And no, a minor annotation wouldn't affect that; you'd need to be using the cards as illustrations to articles about them to make them "content images" rather than "gallery images".)
Third, there are some very good programs/services to significantly shrink the file sizes of both JPEGs and PNGs without visibly affecting quality. I use JPEGmini (a locally-installed program) for JPEGs and TinyPNG (an online service) for PNGs. That will let you significantly expand the number of images you can fit in the same disk space and make your users a lot happier about the download speed and bandwidth.
Fourth, the fronts of postcards tend to be copyrighted images. Unless your collection is antique (so they've become public domain in the USA) or the images are low-resolution images, perhaps with high-rez crops of small parts of the image if necessary, used, again, as illustrations to articles about the postcards (in a way that would inarguably constitute "fair use" under US copyright law) you're not actually free to post them online.