Well, an image can be changed to grayscale in Preview but takes a few steps: 1) First make a copy of the image (just in case); 2) Open the copy in Preview; 3) Either select the editing tool and then the 'Adjust Color' prism or open the Tools menu and pick 'Adjust Color' (see blue arrows in my attached screen capture); 4) Slide the Saturation indicator all the way to the left (see white arrow) - this converted my colorful CD cover art to B&W; and 5) Save the image to the copy and rename as needed.
A more detailed explanation HERE of the same directions. Dave
P.S. if you need to 'batch process' a lot of images, then some type of automated routine or another image app might be a better choice?
Problems like this seem to happen when I'm rushing to prepare material for a talk. Something that I have used many times before is no longer there. And an internet search gives 100 hits all explaining the old way of doing it.
There must be a better way of publicising mods to software??