[Thanks to rhydwen for the following really clear exposition of the security issues concerning the use of GravatarThe profile pictures were going to be provided by a service provided by Gravatar. Harry]
All URLs, for the Gravatar images, are based on the use of the MD5-hashed value of the poster’s email address. So, Gravatar would need to know all the posting participant’s email addresses and would then use the MD5-hash, of those addresses, to form the url for the profile picture.
As Gravatar would serve the profile images for every browser request, Gravatar would know the email address of the poster and also the ip address of all viewers of the post; enabling them to build a picture of the relationship between the poster and the readers of that post.
If another party could guess the posters email domain (the bit after the @ sign), then they could either search a list of email addresses from that domain, to find a MD5-hash match, or even brute force a match to the username part of the email address – deriving the full email address of the poster from their image’s url.
None of the above is the sort of thing a super sleuth would welcome.