Like tabacco said, you don't get to take back things you've said in person, so three days is a luxury.
I think there needs to be a limit to editing. A discussion can become impossible to follow if everyone keeps going back and changing things they said a week ago. There's really only a few good reasons for editing a post (for example, someone just pointed out I posted spoiler material in clear so I've fixed that)
If you've posted something you regret, the chances are anyone who was going to be offended will have seen it after 3 days. If you want to retract your statement they are much more likely to see a new post apologising than notice your old post has changed.
As for compromising pictures, the chances are someone will already have copied it to an e-mail and forwarded them round the world if you've left them a week.
EDIT Editing is good for fixing typos too. But that really is something you should fix BEFORE posting. Hoy Trep. Can you change that "wekk" to "week" in your quote?
