Found a problem with the Snap sample project from Ray Wenderlich site - the poker/playing card images are only for iPhone, and the resolution is not good enough for iPad/iPad Retina!! Need to find a way to create 52+ (plus back, joker, ...etc) card images...
Was looking at some sample codes on the net and thinking about writing a small "auto card image generating" application which allows users to edit their favourite photos and use it as the card image or something like that (any one got this running or available already? Please let me know!). Later found that there's actually quite a few free vector images for playing cards available on the net, cool!
Found 2 so far:
1) https://code.google.com/p/vector-playing-cards/ by Bryon Knoll
http://sourceforge.net/p/vector-cards/home/Poker%20Sized%20Playing%20Cards%20in%20Vector%20Formats%20(.SVG%20and%20.EPS)/ by Chris Aguilar
Personally I think the 2nd one looks better...
Any way, an extra resource for any one interested in making any card games! Enjoy!