Cracking the file open in Notepad++ is... well, exactly as helpful as it sounds, just garbled nonsense. However, the string "OPTPiX iMageStudio 3.0 http://www.webtech.co.jp/" appears repeatedly. That is, apparently, an image optimization program, which leads me to believe that those are indeed supposed to be image files. We just don't have the technology to decode them because they aren't built quite like the ones used in FFXI proper. Feels like a dead end; I'm inclined to say we have what we're looking for, and just have no idea how to use it.

Pity you can't just open up PlayOnline without connecting and having updates pushed, that might have been an avenue to get a look at the files in their original context, though I suspect you'd only be allowed to see cards in your personal collection that way anyway.