Because I can never find this when I need it, here is the location where XCode stores screenshots:
~/Library/Application Support/Developer/Shared/Xcode/Screenshots
To take screenshots for the iPhone, iPad, or iPod Touch, go to Window –> Organizer and click on the device within Organizer. You’ll see a tab titled “screenshots.”
You can also drag and drop the screenshots from the Organizer to Desktop or a folder of your choice.