If you need to quickly resize an image, not by just scaling down, but you want a certain piece of the image. You, could always take another screen-shot of the image or you can use preview to create the new image in a matter of seconds and also get the option to save it in any specific format you wish to.

 

Follow these steps to learn how to do it in Preview :

 

1. Open the image in preview

2. Choose the select tool you want to use to select the portion of the image you want to save as a separate file

 

3. Use the tool and make the desired selection

 

4. (a) Go to Edit -> Copy and then go to File->New from clipboard

 

 

(or)

(b) Press Command + C and then press Command + N on the keyboard

5. Save the new image in which ever format you like -

6. Voila! The selected portion now appears like a new image!