When you download a stock photograph from the internet, it is often a massive, high-resolution file (e.g., 4000×3000 pixels) weighing several megabytes. If you attempt to upload this massive file directly to your website or attach it to an email, it will severely slow down page loading times and clog the recipient’s inbox. Before sharing high-resolution images, you must scale them down. You do not need third-party photo editors to do this; you can instantly resize any image’s exact dimensions and resolution using the macOS Preview application.
How to Resize Image Dimensions
Scaling an image down reduces its physical footprint on the screen, which dramatically reduces the overall file size.
- Double-click the photograph to open it in Preview.
- Look at the top menu bar (next to the Apple logo) and click Tools.
- From the drop-down menu, select Adjust Size…
- A small popup window will appear. Ensure the lock icon next to the “Scale proportionally” checkbox is closed (checked). This prevents you from accidentally stretching or warping the image out of shape.
- In the Width box, type your desired new size (e.g.,
1200). Because the proportions are locked, the Height box will automatically update to match the correct aspect ratio. - Click OK.
The image on your screen will instantly shrink. Press Command + S to permanently save the new, smaller file.
Understanding Pixels vs. Inches
By default, the Adjust Size window usually measures images in Pixels. This is the correct measurement system if the image is going to be displayed on a digital screen (like a website or a PowerPoint presentation). However, if you are resizing the image because you intend to physically print it on a piece of paper, pixels are practically useless.
If you need the image to perfectly fit a 5×7 inch photo frame, click the drop-down menu located to the right of the Width box and change “pixels” to inches. You can now type exactly 7 into the Width box, guaranteeing it will print at the correct physical size.
Adjusting Resolution (DPI)
In the same Adjust Size window, you will see a box labeled Resolution (measured in pixels/inch or DPI). This setting tells physical printers how densely to pack the ink onto the paper.
- For Screens (Websites, Emails): Set the resolution to 72. Digital screens cannot display a higher density than this anyway, so setting it higher just wastes file space.
- For Printing (Photographs, Flyers): Set the resolution to 300. This forces the image to retain maximum density, ensuring the physical printout looks incredibly sharp and crisp rather than blurry and pixelated.