Amazing - that worked, thank you so much.
However, if I set an image from a post as the featured image, that makes it appear at the very top of the post as well as wherever I already placed it in the post. Is it possible to prevent this? I don't want the same image to appear twice in one post, or to necessarily have to have an image right at the top of every post, which is why I've avoided using featured images until now. But perhaps I will just have to change my ways if there is really no way around it.
Having said that, I notice that the featured image comes out cropped, too, so in some cases it wouldn't be a suitable replacement for a regular post image in any case. So I'm really hoping this is preventable...