At work, I wanted exactly that and more: We had an internal page with mugshots of everyone, and from that I compiled small icons so when new people were sending me mail, I could put a face to the name. And while I wanted to share that directory, I also wanted a local directory for pictures of non-colleagues that might email me.
Later, I discovered the MoreFunctionsForAddressBook extension, and it too lets me assign a picture to a contact.
I tried getting in touch with the developer of MessageFaces, but he never replied to my email. So then I did what everyone would have done and added support for all those features myself. Just another side project, I guess. My own, improved version is available here.
1.2.5: - Fixed an error in the preferences URL for gravatars 1.2.4: - Added support for addressbook images from MoreFunctionsForAddressBook extension - Added support for generated default gravatars (wavatar, etc.) - Removed the empty tooltip text from the image 1.2.3 - Allow use of more than one local folder for images