Someone recently asked about the security of a signature image on their web page. They wondered if they should remove it or if there was some way to keep it from being downloaded or spidered and cached by search engines. While I can understand the desire to give that personal touch to a web page I wouldn't publish an image of my signature. That being said, here's some analysis.
I assume we all agree that there is no reliable way to keep a publicly viewable web image of anything, including a signature from being viewed, downloaded, cached, reused, etc. If you disagree, consider that you're not trying to hide it from search engines. You're trying to hide it from people, unscrupulous people specifically. How can you make it public while at the same time hide it from people with questionable intentions? My web-enabled mind reading system is not finished yet, is yours?