Well I remember many people (including Miranda) stating that her sources weren't sure whether they were going to go with the whole "zipline" thing or not (i.e., whether it was viable or not.) In other words, there were doubts on whether Joker should get "scarred" by Batman during the course of the film. Perhaps they never put it into execution and just opted for him already having the scars?
Re: the scar issue, the IBIHDT.com photo shows scars, not wounds. It's hard to put into words, but we're not seeing open cuts or wounds on his face - we're seeing what looks like massive scar tissue that's built up because of what happened to him. Like chunks were taken off his face and what is left is pock-marked, scarred skin.
To look like this in real life, it would take years. I know someone who recently had open heart surgery, and the procedure involves cutting through the chest. To this day, even a year later, the wounds are still reddish-looking and only some scar tissue has formed. To get like the Joker, he would have to go through a massive injury and then let it heal for years to look like that.
Now if this happens overnight in TDK, and Joker is healed in like, a week after he's injured, it's not going to ruin the film for me. I just think it would be cooler if he already had the scars when the film opened up.