Hi,
In my haste, I forgot to get to your actual question.
After adding the settings I mentioned earlier in your user.config. Open your web.config, add a line to the bottom of it and then save it. This will cause the application pool running your website to recycle which will clear it's cache. You should be able to refresh the site and see your new image. If you do not, try clearing your browser cache.
HTH,
Joe D.