to even futher enhance your screenshot workflow in Preview... use this tip that i discovered...
if you want the whole screen just add ctrl to your usual key combo (cmd+shift+crtl+3) to capture the screenshot to the clipboard, then open Preview, and there's an option to make a new document from the clipboard. then just export.
no PDF file sitting on your desktop, the first file that is made is when you export the pic. i much prefer this way, then you're not deleting PDF's etc.
of course you can also add ctrl to the cmd+shift+4 combo, very handy. 😀