OS X’s Preview is the little image viewer that keeps on giving. Preview is so feature-rich that there’s usually little reason to install another image viewer. It can even resize a large batch of images at once.
Resizing in batches makes short work of what would normally be a tedious and time-consuming process, assuming you’re resizing all your images to the same dimensions.
To perform a batch resize, you can either open Preview from the Dock and File > Open your images…
…or, you can select a bunch of images in the Finder, right-click on them, and then choose Open With > Preview.
All of your images should open in Preview’s left sidebar.
Now, you could go through each image, click on the “Tools” menu, and select “Image Size” to resize them one by one. But we have a better way. First, select all the images by clicking on the “Edit” menu and then “Select All”, or using the keyboard combination Command+A.
Now you will see that all your images are highlighted, which means they’re selected.
Now, click on the “Tools” menu and select “Adjust Size…”.
Since our images are varying sizes, the width and height values will tell us they have “Multiple Values” but we know that our width needs to be 325 pixels for all of them. So, we enter this number and then click “OK”.
We’re not done, however. In order to make these values stick, we need to save our images. Again, we don’t have to save each one separately. Since all our images are selected, we can go ahead and click on the “File” menu and choose “Save”, or use the keyboard shortcut Command+S.
Once saved, you will notice that the “Edited” notation in the titlebar will disappear.
You can use this trick with any other changes you make to multiple images in Preview, so you can rotate, adjust the color, and more. But for us, resizing is bar none the most useful option for our purposes. Undoubtedly this will be a big time saver for many folks who use a Mac and the Preview app.