How To Resize A Photo On Mac In Preview

How to resize a photo in Preview on a Mac

There are a bunch of ways to resize a photo, but probably the easiest for Mac users is to just use the bundled software (Preview) that came with your machine! In this quick tutorial we'll take a sample photo and show you how to get it to the specifications you need for web use.

Why does changing your photo size matter? The biggest reason would be for website speed optimization. The smaller your photos file size is, the faster your website will perform. We're talking about file size here, not how big your image is. You can still have a beautiful and large image with a small file size. The general rule of thumb is no larger than 256kb for an image. You can find your image's file size by right clicking on it and selecting "Get Info".

If you're a Windows user and want to know how to resize your photo in Paint, read our how-to post on that here.

First get your image.

For this example we went to Pexels (wonderful source for royalty free stock images) and pulled a random photo.


TIP: Duplicate your image first. All of these adjustments we'll be making are permanent so it might be a good idea to save your original, large, high resolution photo in case you need it for a different application later. To do this just Right Click on the image and select Duplicate.


Royalty free Pexels photo that we'll be resizing on a Mac computer in Preview.

Click on the image above to see it full size. The resolution isn't too bad for only a 100kb file!



Open Photo In Preview

Double click on your photo to open it in Preview on your Mac. Or open it's duplicate if you chose to go that route.


Let's adjust your photo's size

Go to Tools > Adjust Size


With your photo open, go to Tools then Adjust Size...


Adjust your resolution first

WRONG! Don't do this. Adjusting your photo's resolution to lower it's file size is actually a myth. Pixels/inch actually has nothing to do with how large your photo's files size is or how it will display on a screen. It only pertains to how that photo will print. Instead of lowering it's resolution all you need to is adjust your photo's dimensions to compress it a bit. Our example photo comes with a 6,000px by 4,000px dimension, and a 240 pixels/inch resolution, for a whopping 16.3MB file size! We're gonna need to bring that down.


Photo with original resolution and dimensions. 5MB size now.

Here we adjusted the resolution down to 72 px/in and it didn't change our file size.


Next adjust the dimensions

Here's where the actual magic happens. Make sure the little "lock" icon or "Scale Proportionately" is active. If you don't do this you might end up with some severe and screwy distortions in your photo. You don't want that. If you don't make sure that lock or scale proportionately is active. Or in the words of South Park, "You're gonna have a bad time."

You want to change just the longest side of your photo to make sure it doesn't get wonky. There are a lot of opinions on what the optimal dimensions for a web-use photo are, and a lot depends on how you're going to use your photo. If your blog section of your site only displays at 700px wide, there's absolutely no need to go wider than that with your photo. Just for generalization, unless your photo is going to be a large, full-page background, 1,050 pixels at it's longest dimension is about the max you need.

Another reason why sometimes it's good to make a copy of your original photo before editing the size. It's nice to have a backup in case you go too small with your adjustments you can always go back and try again.


Adjusting the longest size of the photo automatically adjusts the shortest side to keep dimensions proportionate.


BOOM! Dropping just the dimensions down to 1,050px X 700px brought our file size from 16.3MB to a very conservative 108KB. Perfect for web use and an extra-fast page loading speed.


Close the window or click save

That's it! You're golden! Pat yourself on the back, play "Eye Of The Tiger", tell your boss (or yourself if you're an entrepreneur) it's time for that big raise, go high-five Joe in accounting. You earned it. 

If your photo's file size is still pretty big, over 500kb, it might be a PNG file instead of a JPG. If preserving any of the transparent aspects that come with a PNG file aren't important to you, go ahead and export your file from Preview as a JPG at about 80% quality. File > Export> JPG.


TIP: Run your photo through JPEGmini

JPEGmini is a neat little application that compresses your image file size even further without any noticeable negative change to your photo's appearance. Their paid version costs a little bit of money but they offer a free trial and the first 200 or something photos you run through the app are completely free. Pretty sweet.