Oops, sorry. The mind has trouble believing anyone prefers using a Windoze server in this day and age . . .
I assume you meant keep arachnids out of your image folder?
User-agent: *
Disallow: /
That's the basic format, so if you want to disallow certain spiders from a certain directory, you'd do this:
User-agent: Slurp
Disallow: /images/
More info here:
Yahoo! Help -