Similar to how Google slowly unrolled Google Talk within Gmail, users of the popular webmail service will be experiencing a new feature shortly. "Gmail Pictures" is much more akin to avatars than pictures, however. The images will appear when hovering over a user's email. It is unclear at this moment whether or not a person must be in your contacts list to make use of the picture. You can spot some screenshots on Gmail's What's New page.

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.
Source: Google