Thanks everyone for answering, but hasn't quite got to the root of my problem.
Yes, I can see how that works with clips in an event: I reject them so they are in iMovie's trash, then when I view rejected and "move to trash", they go into the actual computer's trash.
But this doesn't seem to work with stills deleted from a project, which is where I'm trying to do it.
If I delete a still clip from the project, and then go to show rejected and there is nothing there.
Howver The still clip is visible in my project if I go into the finder, so I can delete it from there, but this seems to be a bit of a palaver, I would think I should be able to do this in iMovie.