This tiny tutorial describes how to capture a screenshot of a scrollable iframe in most modern browsers.
Change screenshot storage location under Mac OSX you have to run the following command on a terminal:
$> defaults write com.apple.screencapture location ~/Pictures/
$> killall SystemUIServer
After running the commands above, your screenshots are stored in the Pictures folder of your users home folder. The article screenshot shortcuts for Mac OSX describes how to take screenshots via shortcut on your Mac.