Open the picture 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 file
Open the picture in Photos
Click Edit
Click Crop
Adjust the crop frame
Click Done
Open the picture in Pages, Keynote, or Numbers
Select the image
Click the Format button
Choose Image
Click Crop
Adjust the crop area
Press Return or click Done