Or you can use ImageMagick if you have it installed.

The following will resize an image if it is larger than the given size while retaining the image proportions. If you always want to resize the image (ie. enlarge an image if it is smaller than the given dimensions) omit the ">".

convert -resize "100x100>" image.jpg image-thumb.jpg

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i am jack's amusing sig file

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This is true, but patniemeyer's suggestion doesn't require installing ImageMagick (sips is an interface to Apple's Image Events processing system).

Just type sips -Z 100 image.jpg --out image-thumb.jpg to achieve the same result as your command.

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