If you need to capture an image of a full web page without having to do the scroll-cut-paste-repeat dance you should have a look at Paparazzi. Paparazzi uses the WebKit framework to create nice full length captures of web pages.

If you need to do this programmatically for a large number of sites there is a nice library called webkit2png by Paul Hammond. It is a Python script utilizing the PyObjC bridge .

Update: There is also a cheap commercial alternative to Paparazzi called Red Snapper. For $8 you get a button directly in Safari to do a capture. Worth checking out if you don’t want an extra app on your system.