This resizing skill has been somewhat overlooked because it’s a bit buried away in the menubar under ‘Tools’. But, in my usage of it, I’ve found it to be flexible and useful – surpassing shareware apps such as EasyBatchPhoto, and rather lacklustre freeware apps such as iZoom or PhotoDrop.

Before you start resizing with Preview, though, be aware that Preview will overwrite your current files, so have back-ups ready in case you don’t like the results. So, here’s how to get Preview.app to resize a bunch of photos…

1. Open the images that you’d like to resize, in the Preview app.
2. This is important: select one image, so that the thumbnail in the sidebar is highlighted blue, then do a ‘Select All’ (hit: Command/Apple and A) so that all the thumbnails are highlighted in blue.
3. In the menubar, navigate to ‘Tools’ and then click on ‘Adjust Size…’ (see image, below):

4. Now the Image Dimensions box appears, and you can now choose how you’re going to scale all your photos. I usually take my overly-large 3648 x 2432 photos down to a more manageable 2400 x 1600, which also reduces the file size by nearly half, without any perceptible loss in quality. (see image, below):

5. Once you’ve picked the right dimensions, hit ‘OK’ on that Image Dimensions box.
6. Wait for the images to be resized, as indicated by the progress bar. This could take a few minutes for very large files, or hundreds of images.
7. Nearly finished… But, wait, because the new images have not yet been saved. Navigate to ‘File’ in the menubar and click on ‘Save All’. A progress bar appears as the images are saved. (See image, below):

Ta-da…!

OK, all done. The only caveat is that landscape- and portrait-oriented photos and images need to be done separately, which can be a bit of a pain.

So, that’s how the Preview app can save you money and get your photos resized with no loss of quality, saving you time on uploads, and space on your hard-drive.