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
Select Crop
Adjust the crop box
Click Done
Open the image in the Finder
Press Spacebar for Quick Look
Click the Markup icon if available
Crop using the selection tools
Save the changes