How to crop photos with Preview on a Mac:

Open the image in Preview.

Open the "edit toolbar" (which is different from "the toolbar").
	View -> Open Edit Toolbar

Click on the Select button on the toolbar.

Click Rectangular Selection in the drop-down menu.

Click and drag your mouse over the part of the image you want to keep.

In the toolbar, click Tools > Crop.

To save the result, use the File -> Export.. menu item.

(Extracted from a web page that hides its URL in Safari.)

Warning: Preview sometimes refuses to close the window used  for  the
above  editing;  it may only close it if you "Revert" the file, which
undoes your cropping.  When it does this, you must export it  with  a
new  name, then close the window (with Revert), and copy the new file
to the old one.  Inquiries about this got the reply that it's working
the way it was designed to work.