Do so by double-clicking on the blue Preview icon that looks like overlapping snapshots, then clicking on File in the menu bar and Open... in the drop-down menu. Select a file in the dialog box and click on Open.

Preview is Apple's native image viewer app that's automatically included with most versions of Mac OS.
 It's in the menu bar at the top of the screen.,, Select new dimensions or "Custom" from the "Fit into:" menu.

If you select "Custom," enter the dimensions in the "Width:" and "Height:" fields, and/or the number of pixels per inch/cm in the "Resolution:" field.
Enlarging an image may result in pixelation and reduced quality.
, The image will be saved with the dimensions you specified.

To save the image in a different format, click on Export As…, then click on the "Format:" drop-down menu, and select an image format:

JPEG - best for photos on the web. Compact files.

JPEG-2000 - high quality, good compression. Smaller files.

OpenEXR - best used to compress video files.

PNG - best for graphics and small Web files. Larger files.

TIFF - best for editing and storing images. Larger files.
Open a photo in the Preview app.
Click on Tools.Click on Adjust Size….
Set a new size for the image.Click on OK.
Click on File in the menu bar, then click on Save.
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