Speaking as someone who could write a book titled "Everything I Know About Copyright Law I Read On The Internet" (but won't), fair use is one of the ugliest areas of copyright, but the broad things I think you should know are:
1. Fair use allows for the use of (a portion of) a copyrighted work to be used in the context of commenting on that work. So "Here's some discussion about Dominion cards" should be fine for making use of images of the cards.
2. Generally, the amount you're allowed to use under free use is the amount that you need to make that commentary. So for "Check out how the art for Island, Pirate Ship and Native Village form a triptych!" then you probably only need fairly low-res images of the three together, while "See how the road signs on Crossroads point to game-related locations like Essen!" would need the higher-quality version to see the details.
3. Whether or not you have a valid claim of fair use, your likelihood of being able to use part of the copyrighted work in something on a site like Facebook or YouTube will tend to be correlated with how many people could lay some kind of claim on the work and how many of them are nice people. As pointed out, Donald and Jay are generally pretty cool, and I suspect the card artists are too, so as long as you're not competing with them commercially in some form (like printing off your own copies of the cards) then you should be fine.