A good source: planetff12.de

You also have a lot of good screencaps there. Only issue: they often have logos somewhere on the pic.

Otherwise, ffxii.net is a good source for artwork.

I'm currently working on my own ff12 screencaps; I'm quite new here though, and I'm not sure if I can post samples here, or link to the site where they will end up: don't want to post a shameless plug if it's again the rule ;-)