A couple of people have posted different methods of loading/using additional images, which they think are better or easier. It's a matter of taste and style. There are also some extensions to the display of additional images.

Your image description sounds like you are doing it right, so I wonder if your template has some custom tweak that disables them. Commercial templates have been known to do that sort of thing, overriding admin settings to remove the ability to use some stock feature that the designer doesn't want in their layout. What template do you have? Seeing your site live might help us advise.

What happens if you put images in the base /images/ folder?