Image 404s

I have just started using Radiant (which is just great, by the way),
so apologies in advance for what may be a very simple question: the
server log shows various 404s to image files: button.png and new-
snippet.png are two examples. My setup is on locahost, for testing, so
the 404 refers to a (for example) this url:
There are quite a few image files in /images/admin, but not