Apple's Preview app has come quite a long way over the years. Once used almost exclusively for reviewing PDFs or looking at pictures, Preview in OS X Yosemite has become a resource-rich photo editing app. Not only can you crop, rotate and resize images, it also includes advanced options for marking up photos or adjusting color and resolution. When editing a photo, remember that Preview automatically saves changes. If you want to keep the original photo intact, make a copy before you start editing it.
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Double-click any image on your Mac. By default, Apple opens images in Preview. If you've changed the default app for your images, right-click the file, select "Open With" and click "Preview." Crop the image simply by dragging the cursor over the area you want to retain, then press "Command-K."
Click the "Rotate" icon to rotate the picture 90-degrees counter-clockwise. Pressing "Command-L" or "Command-R" also rotates the image.
Click the "Tools" menu for access to all of the rotate options as well as to flip the image horizontally or vertically. You can also adjust the image size and color, or select the "Annotate" option to add shapes, text or speech bubbles to the image.
Select "Adjust Size" from the Tools menu to change the image dimensions and resolution. The "Fit Into" menu offers several common dimensions, ranging from 320 by 240 pixels to 1920 by 1080 pixels. If you're changing the width or height, make sure the "Scale Proportionally" option is checked if you don't want to stretch the picture.
Select "Adjust Color" from the Tools menu for Preview's advanced color options. You can change the color levels, for example, by dragging the sliders at the top of the Adjust Color window. Sliders also control the exposure, contrast, highlights and shadows. To change a color image to black and white, drag the "Saturation" slider all the way to the left and then adjust the contrast and exposure as needed.
Select "Show Markup Toolbar" from the View menu to reveal the icons you need to draw or write on the picture. To add a caption, for example, click the "Text" icon and then drag the cursor across the picture to add a text box and type your words in the text box. Change the text formatting by clicking the A-shaped "Format" icon.
Save your image as a new file by selecting "Export" from the File menu. From the Format menu, select an image type, including JPEG, TIFF, PNG and PDF. Adjust the image quality as needed by dragging the "Quality" slider. Note that Preview displays the file size so you can find the right balance between high quality and a file size you can quickly post online or email to friends.
Click the "Revert To" option under the File menu if you don't like the changes you've made to an image, or if you forgot to save a copy before editing it. Preview lets you go back to the last saved version, last opened version, or view all versions.