Not having the image provides even less certainty of security. You assume that the image being there will make people less likely to check other aspects of the

Not having the image provides even less certainty of security. You assume that the image being there will make people less likely to check other aspects of the site, like the URL. But consider the average user. Spoofing and phishing attacks work because people don't check these things. The security image is difficult to spoof and is more likely to clue average users in to attacks. Therefore, it is useful as a security device and is not worse than not having it.