Del, this worked for me. I start with an image in full view. I
create a rectangle and position it over the image. It's probably
transparent, but it works just the same if it's opaque. (On my first
try, I tried to draw the rectangle perfectly aligned. I may have
tried twice, but at any rate I had a transparent rectangle exactly
matching the image's border, so I couldn't tell it was there, and that
caused real mystery in the next step.)
So, the rectangle is selected. Now hold down Shift and select the
image as well. Both must be selected; it doesn't matter which you
select first -- the "top thing" Dave mentioned is top in drawing
order. Usually that means most recently drawn, unless you've changed
the drawing order with the "Back", "Backward" etc. actions.
Now you should be able to do Format -> Images -> Crop and get a red
group border and semitransparent red fill over the cropped image. If
your mistake was to not select both items, the "Crop" action would be
grayed.
You will discover that it's not just a cropped image you have -- the
whole history, the original image, the cropping rectangle, and the red
group apparatus are all there, so you can Undo or change your crop by
simply Ungrouping it.
Larry
Larry