Strange, the shots out of my 620 are usually VERY crisp and sharp. Maybe you got a bad camera?
Anyways, as far as i know you can't do it in camera. I just looked through all the menus to verify and couldn't find a way, which would be consistent with all/most of the Powershot series that i've ever owned (A80, A510, A520, A620).
I would recommend using something like Photoshop or Elements or GIMP (Free!) for serious jobs, and for quick jobs using free tools try something like Picasa.
I just bought this 4 days ago upgraded from my 3.2 Sony Cybershot. But the Canon's pics are very soft and not as sharp as the Sony's. They almost seem blurry. I know it's not from the camera shaking and I've been playing with the settings but can't figure it out.
Could it be the lens? Even in Auto mode I'm very disappointed on the image sharpness, I have to delete most of them because they are too soft. The only time I get a decent picture is when I use the MF mode but then the rest of the image is blurry.
Thanks in advance!