Preview will not add arrows to photos

I'm trying to explain an engineering problem to a friend and have downloaded a photo to which I am trying to add arrows using 'annotate' in Preview. I click on the arrow icon in the dropdown menu, I get two blue circles in the middle of which is a small green dot. No arrow, no 'drag and align' line, no pointer. Just this small green dot that sits there, doesn't get bigger or smaller, and won't let me draw an arrow. I can align the two blue circles as if I'm lining up a straight line, but there's no arrow, just this green dot. What am I doing wrong?