Go to Finder and click twice on the image you wish to edit.

Preview will then be activated, and your image will appear at the top left side of your page.

I have just chosen a random image to demonstrate this!

You will see the Tools bar right at the top of your page, where the blue arrow is pointing.

If you want to keep your original image as it is, the you need to duplicate it before you do anything else. To do that, click on File > Duplicate.

Now you are ready to Edit your image.

Next up. Resize Your Image