I've mostly found the spoilers to be so subtle that I don't realise they're spoilers until I've read the scene in the book, at which point I go " Snap! So that's what that card is! I thought it was just a generic picture...."

Sansa examining the black amethysts given her by Ser Dontos was one, Arya whacking the crap out of trees while reciting her hated names was another. I completely missed the fact that half of Tyrion's nose was lopped off until I got to that part in the book (I just thought the artist was making the Imp look mad ugly).

The Whitebeard/Selmy thing was a quite a heavy spoiler, though, but serves me right for browsing Dabbler so thoroughly before actually finishing Clash of Kings. They seemed to learn from that by not spoiling Lady Stoneheart in the same way...

"You Killed the Wrong Dwarf" is another one that reeks of spoiler.

Not art, but the Core Viserys returning to your hand rather than being killed or discarded had me waiting for him to somehow be revived throughout the whole series.

There are plenty of cards around with spoilers, and there's no way that could have been avoided.

I remember upon seeing Stannis's Cavalry, a card showing cavalry fighting men in the snow, and then the flavor text reading "and yellow, so much yellow, yellow banners with a red device, whose arms were those?" when I realised Stannis was sending army to The Wall. Also The Wall (TWoW) clearly shows Stannis with Melisandre atop the wall. Also Stannis Baratheon (AKitD) shows him with the wall in the background. The Epic Battle event The Battle at the Wall is also a spoiler on this.

My favorite is the dual trait Jon Snow. I had only watched the first season of the show, and hadn't started the books yet, so, when I read the card and saw Nights Watch. Wildling. In his trait spot, I had the o.O reaction, until I read CoK.

While it really is not much of an overt spoiler, finding out that the Horn of Dragons was actually a legitimate reference to the books was awesome, especialy considering the ability itself is exactly what the Crow's Eye wants to use in the books. 

I would suggest reading until you reach Dance with Dragons before picking up the card game again.