Unless you have installed an image manipulation program such as Adobe Photoshop or the GIMP, images should by default open with the Preview app on a Mac. From the Finder you can get the image resolution and size information by using the command-I (that’s the letter ‘i’ as in information):

Using the Get Info command in the Finder

Here we can see that this image taken with an iPhone is 3.5 mb. This is a much greater resolution than is necessary for the web. We will reduce it.

METHOD #1

Open the image in Preview by double-clicking on the image. The easiest (and fastest way) to reduce the size of this image would be to choose “Export” from the “File” menu:

Choose the “Export” command from the “File” menu

You will be presented with a dropdown menu:

The Export dropdown menu in Preview

There is a quality slider menu and underneath you can see the file size. Use the slider to reduce the file to a more manageable size (usually less than 1 mb). Save the image. Open the image again to ensure the image has not become pixelated.

METHOD #2

For a finer control over the quality of the image, once you open it in Preview, go to the “Tools” menu and choose “Adjust Size…”:

Choose the “Adjust Size…” option from the “Tools” menu

You will be see a menu with the dimensions and resolution of the image. For the web, 72 dpi is sufficient:

Dimensions and resolution of the image

The default on my computer was inches. Change inches to pixels in the right-hand dropdown menu. You only have to change one dimension, the height or the width, WordPress will keep the aspect ratio for you. For the web, leave the resolution at 72 dpi. You will see a very large number for the dimensions in pixels. Depending on the ratio of the image, I chose 800 pixels as the width of the image:

Change the dimensions to pixels

As you can see, with these settings, the image size was reduced from 3.5 mb (3,584 kb) to 180 kb. Choose “OK” to keep your new settings. Save the image under a different name (in case the new resolution is too low and you have to resize it again) and reopen it to make sure there is no pixelation. Remember that since you have already adjusted the size of the image, you do NOT want to reduce the quality again when exporting this image (see method 1 above).