Excerpted from Mac OS X Mavericks: Peachpit Learning Series by Lynn Beighley
You can crop any photograph or other image very easily in Preview. Caution: When you crop an image, the part you remove is gone from the original image forever. Before cropping an image, you might want to make a duplicate of that file.
Open the image you want to crop. Choose View > Show Edit Toolbar and then choose a Selection tool from the left side of the Edit toolbar. You can crop the image into a circle, oval, or rectangle, or you can use the lasso tool to draw a custom shape. After making your selection, click the Crop button. (It shows up right after you select part of the image.) You can resize the cropped area by dragging any of the tiny round “handles” that appear on each corner and side (you don’t get handles on a lasso selection). To move the cropped area to a different part of the image, press in the middle of the cropping box and drag. To cancel, press Esc or click outside the selected crop area. When you’re ready to crop, choose Crop to Selection on the Edit toolbar, or press Command-K.
I hope this helps you out in your productivity work flow. Another great tip showcased on Tippy Tuesday.