Gmail's "What's New" page has changed and tells us of a new feature that will be launched shortly -- Gmail Pictures. The new feature basically lets you see who has emailed you -- almost like an avatar in instant messengers.
The new feature won't be available for all users at once, it will be rolled out slowly just like the chat feature did.
From the Gmail Picutres help page:
What are Gmail pictures?
Gmail pictures allow you to put a face to a name, by showing user-selected pictures when you roll over a name in your inbox, Contacts, or Quick Contacts list. You can choose your own photo for yourself and for your contacts, suggest a photo for a contact to use, or see what your friends have selected for themselves.
Gmail pictures can be configured to show to all users who you email or who email you, or they can be limited to only those users who are able see when you're online. You can see a full list of users who can see your online status from the All Contacts tab of your Contacts page – they'll be listed with a colored ball next to their name.