Note that this tag only advises the browser to avoid caching the HTML page, it does not tell the browser to avoid caching the graphics that may be referenced on the page. If you access the Web through a proxy server, as many large companies do, it is also possible that the proxy server caches pages and graphics in a way that is beyond the control of either the browser or the server. In other words, this isn't perfect.
If you have a page that changes graphics very frequently, the most reliable approach is to deliver the graphics through a CGI program, using a tag something like <IMG SRC="http://www.domain.com/cgi-bin/getimage.exe">.