How to Resize Images in Apple’s Preview
Sometimes you might need to reduce the width and height of an image. This might be in order to reduce its file size, or just make it more manageable in whatever application you’re using for your design.
Apple’s Preview application provides an easy way to do this.
1. Load the image in Preview
On a Mac, if you double click a standard image file such as a JPG or PNG, chances are it’ll open in Preview by default. If it doesn’t, right-click it, hover over “Open With”, and select Preview from the list of applications.
2. Adjust Size
With the image now open in Preview, find the Tools menu along the top bar, click it, and select “Adjust Size..”.
3. Set Your Options
Change the Units
You can set the size in a range of units:
Pixels
Percent of the current size
Inches
Centimetres
Millimetres
Points
Scale Proportionally
By default, Preview will keep the width and height proportional. If you change one, the other will automatically update to preserve the aspect ratio. But, you can turn this off so that you can change the width and height independently. Either click the little padlock symbol to ‘unlock’ it, or uncheck “Scale proportionally”.
Resampling
You might have noticed that second checkbox, labelled “Resample image”. It is checked by default. Resampling means that Preview will can adjust the hight, width, and pixel resolution independently. That is, if you change the width, the new image will have the same pixel density as the old image. If you’ve made the image smaller, pixels will be removed. If you’ve made it bigger, Preview will add pixels and make a best guess at what colour and level of transparency they should be.
If you uncheck “Resample image” however, you can no longer set the width and height in pixels. You can only set the size using the physical units such as inches or centimetres. The pixel density will update accordingly, so that there are always the same number of pixels. If you make the image smaller, the pixels actually get smaller. If you make the image larger, the pixels actually get larger.
Consequently, there are always the same number of pixels in the image, and the file size won’t actually change. To my mind, I can't see why you wouldn’t resample the image, but there we are.
In Summary
Preview provides a quick and easy way to change the size of your images, and is certainly a good deal quicker than loading something up in Photoshop or similar for this relatively simple task.