On an Apple OS X system, you can reduce the size of an image by using the sips command with the
-Z size parameter where size
is the maximum value you want for either height or width. E.g., suppose I
have a PNG file named system_preferences.png", which
is 1,560 pixels wide by 1,604 high. The image height is greater than the image
width and I want the maximum dimension to be 780 pixels. By opening a
Teminal window - the Terminal utility is found in the
Applications/Utilities directory - I can get a command line interface
(CLI), aka a "shell prompt", that will allow me to enter the command
sips -Z 780 system_preferences.png. The command will ensure
that neither dimension will be greater than 780 pixels. In this case the
height will be set to 780 pixels and the width will be adjusted to maintain
the current aspect ratio, so the new dimensions will be 758 pixels wide
and 780 pixels high.
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