You can assign an image to each of your accounts during the account setup or change the image at any time in Menu > Accounts. These avatars will help you determine which account your messages belong to in mixed views and search results.

These avatars are currently only stored locally they are not the same as contact avatars downloaded from the domain or Gravatar.

To change the avatar your contacts might see when they receive your messages read this FAQ article: How do I change my own email account avatar?