Open the photo or image in Preview
Click Tools
Click Rectangular Selection
Drag to select the area you want to keep
Click Tools
Click Crop
Open the image in Photos
Click Edit
Click Crop
Adjust the crop box
Click Done
Open a screenshot in the Markup tool
Click the crop icon
Adjust the crop area
Click Done
In Pages, Keynote, or Numbers, select the image
Click Format
Click Image
Click Mask
Adjust the crop frame
Click Done
Use the keyboard shortcut Command + K in supported apps if available