application.hbs

{{gravatar-image email="cavneb@gmail.com"}}

gravatar-image.hbs

<img {{bindAttr src=gravatarUrl}}>

gravatar_image_component.js.coffee

App.GravatarImageComponent = Ember.Component.extend
  tagName: ''
  email: ''
  size: 200
  gravatarUrl: (->
    email = @get 'email'
    size = @get 'size'
    "http://www.gravatar.com/avatar/#{MD5(@get 'email')}#{?s=@get('size')}"
  ).property('email', 'size')