You can add one of your own at users > your profile, but that can be problematic. My suggestion is to go directly to http://en.gravatar.com/ while logged into wordpress, click on "my account" at upper left and select "add an image" and upload your pic. It will typically take anywhere from an hour to as much as six for it to show up everywhere. Also, make sure and clear your browser cache and cookies every once in a while until it starts showing up. Sometimes browsers will hold onto the old graphic until you do.