Google has just announced a change coming to the Google default avatar. Google account owners with Google+ enabled have the ability to add a picture to their account profiles, which helps clarify which account users are logged into. In order to extend that benefit to account owners who have not added a picture to their profile, Google is replacing the default Google account avatar with a custom avatar showing the account owner's first or last initial, depending on the structure of the account owner's name. If initials are not available, a color will be used.
For more detailed information, see Change to default avatar for Google account owners.