By default, you can simply double-click an image file and it will open in Preview. If you’ve changed your image file associations, you can Command-click or right-click on an image file, point to Open With, and select Preview.
You can also open the Preview app from the Applications folder, Launchpad, or by pressing Command + Space to open Spotlight Search and searching for Preview. From Preview, you can open the image file directly. Or, with Preview open, you can click File > New From Clipboard to import an image file from your clipboard. You can then edit the image and get it back onto your clipboard by clicking Edit > Copy.
Once you're on the preview of the photo, select Tools > Adjust Size to bring up the Resize dialog, which will allow you to resize the image. It supports many measurement units, including pixels. By default, it will resize the image proportionally, maintaining the original aspect ratio to ensure the resized image doesn’t look stretched.
