Open the image in Preview
Click the Markup toolbar button
Select the rectangular selection tool
Drag to select the area you want to keep
Go to Tools
Click Crop
Save the image
Open the image in Photos
Click Edit
Click Crop
Adjust the crop box
Click Done
Open the image in Finder with Quick Look
Press the Space bar
Click Markup
Use the selection tool to choose the area
Click Crop
Save the file