For bigger contrast and easier flush-detection some online casinos offer 4-colored cards, where club is green instead of black and diamonds are shining blue instead of red (see pic attached).



Betting on a false-flush is no fun, believe me

To help with flush detection, one could also take these cards http://commons.wikimedia.org/wiki/Category:OpenClipart_playing_cards and use a different variation for each suit.
Replacing the card images in pokerth is very easy actually, as you must know. I use a mix of the french svg cards by david bellot and scanned Dutch cards for J, Q, K and A because I love the design of those (check http://www.gamesmuseum.uwaterloo.ca/VirtualExhibits/Playing%20Cards/decks/dutch/index.html to see what I mean , it's very similar the first deck depicted but you can see the suits better on the modern Dutch deck)

Regards,
heyn