You can use Preview  to resize images quickly and easily.

1. Firstly, locate the image that you want to resize.
   
    
2. Preview  should be the default application for opening images, but just to be sure, follow these steps:
   1. Right-click (or Control-click) on the image file.
   2. Select the Open With submenu.
   3. Select Preview from the options that appear.
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3. Once the image has opened in Preview , go to the Tools menu, and select Adjust Size...
   
    
4. This will bring up a dialog box where you can set the new dimensions for the image.
   
    
5. The Fit into: drop-down menu allows you to select from a number of preset sizes (in pixels), or you can select Custom to enter your own preferred dimensions.
   
    
6. If you choose to enter your own custom dimensions, you can select your preferred unit of measurement from the second drop-down menu, next to the padlock symbol. The most useful options are:
   • Pixels, which allows for easy comparison to your desktop size; and
   • Percent, which allows you to decrease or increase the size of the image relative to its original size.
     
      
7. If you need to resize the image to a shape that is out of proportion with the original, you can just uncheck the Scale Proportionally checkbox. This will allow you to set different values for the width and height without it automatically adjusting.
   
   In this example, the width and height have been set to 50% of the original.
   
    
8. Once you're happy with the changes you've made, click OK.
   
    
9. You can now see the newly resized image. Preview  automatically saves the file whenever a change is made, so once you're done, you can safely exit the application.