You inspired me to do a bit more digging on google. Reading around I strongly suspect Norton, even though I was pretty sure I'd turned it off. When I next have the machine in front of me I'll more carefully verify it is off, indeed uninstalling it completely if I need to. One of the reasons that I am particularly suspicious is that apparently Norton (in common with other stupid software) looks at the image size and the path name to determine if it thinks it might be an advertisment. The images were odd sizes but they are in a directory called /images/banner on at least one of the problem pages and apparently "banner" is a bad word as far as Norton is concerned, so it may well be that.
I'll let you know.