Caesium by Matteo Paonessa is a well-known open-source app that will help you change the size of the images, without losing much of the quality and preserving the original format. Unfortunately, Caesium for Mac has not yet been released, therefore, you will have to use other image resizers for Mac. Here is a list of alternatives to Caesium for Mac.
Feel free to add any alternative to Caesium for Mac that you know of.
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