There is no need to install a third party tool for this. The inbuilt "Preview" Application is enough.

  1. Open The Image using Preview App. Right click on your image and click "Open With" and select "Preview".
Opening Image with Preview App

2. Once Opened, Click on "Show Markup Toolbar" Icon on top right hand side.

Opening Menu Bar in Preview App

3.  In the Menu bar, Click on Rectangular dashed line . This is a selection tool to crop your image. There are other selection tools available in the dropdown. You can choose them based on your need. For now, we shall fix to the "Rectangular" one.

4. Now click on anywhere on your image and drag it till the point you want to crop.

Cropping An Image In Mac

5. Once selection is done for cropping, either you can Click "Crop" button on Menubar or  Press the "Command Key ⌘" +"k".

Cropped Image. 

That's it. You are done with Cropping!! Remember cropping will change the original image.So, better to have a copy before cropping.

Credits: Image by Sven Lachmann from Pixabay