Mac OS X's Preview utility lets you open, edit and save images in the JPEG format. If you need to make an image smaller or decrease its total file size, you can use Preview to perform one or both of these actions on your JPEG image. Reducing an image's size allows you to specify a custom dimension or use a percentage value. Decreasing the JPEG's file size lets you select a lower level of image quality and results in a smaller file size.
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Click your Mac's "Applications" folder icon in the dock. Click "Preview" to launch the program. Click "File" and select "Open." Select your JPEG and click "Open."
Click the "Tools" menu and select "Adjust Size" to reduce your image's dimensions. To specify a percentage value of the original photo size, click the drop-down list to the right of the width and height settings and select "Percent." Select "Pixels" from the drop-down list if you want to specify the image size in pixels or "Inches" to specify a size in inches. Type either the percentage, inches or pixel value in both the width and height settings boxes. The bottom of the window will show the resulting file size in megabytes.
Click "OK" to return to Preview's main window. Click "Unlock" when Preview asks if you want to make the selected changes to the image.
Click "File" and select "Export." Type a name for the updated JPEG and select a save location. Click the "Quality" slider and move the slider to the left if you want to reduce the image quality. Drag the slider farthest to the left for the lowest image quality and smallest file size possible. Click "Save" to export the image with its new dimensions and quality level.