The selector tool should be immediately active and visible by default with a picture open, so draw the rectangular selection around the region to crop down as usual.

Select the image to crop in the Finder then hit Command+O to open it in Preview (assuming Preview is the default image viewer application).

This is an efficient trick to use if you plan on using the crop function often and want to speed things up:

You can even speed up the cropping process by using keyboard shortcuts throughout the task, here is basically the same process as started from the Finder. Cropping Images Faster in Preview with Keyboard Shortcuts Remember that by doing this you’re also indirectly resizing an image by reducing the total amount of pixels contained in the image file, but unlike the bulk resize functions you can not crop in a similar bulk fashion with groups of images in Preview because it requires a unique selection. The video walkthrough below demonstrates cropping an image from this wallpaper post: You can draw the rectangular selector to whatever size you want and the crop function will take care of the rest. See, that was easy right? You now have a cropped image. Go to the “File” menu and choose “Save” or “Save As” as desired to save the cropped version of the image.Press Command + K keys to crop the picture, or go to the “Tools” menu and click the “Crop” option to complete the crop, the image will immediately crop down to the section that was drawn within the rectangular selector tool.Draw the desired rectangle on the image to the region of the picture you wish to crop.Now choose the “Rectangular Selection” tool, it’s usually set by default but you can double-check this by choosing it from the left-most pulldown menu of the Editor Toolbar.Click the “Editor Toolbar” button, it’s located on the right side of the general image toolbar and looks like a toolbox or a little pencil.Open the image file you want to crop into the Preview app in Mac OS X.Cropping a Picture with Preview in Mac OS X The rest of the process is very simple and can be done quite fast, particularly once you learn how to use the tools and master some keyboard shortcuts. To try this yourself, you’ll want to have an image handy that you can crop down, and just about any version of Mac OS X.

How to crop a photo in macbook