Gravatar has you covered--they allow you to specify a default image URL to display if the user doesn't have a Gravatar. From their docs:
If you'd prefer to use your own default image (perhaps your logo, a funny face, whatever), then you can easily do so by supplying the URL to an image in the d= or default= parameter. The URL should be URL-encoded to ensure that it carries across correctly, for example:
<img src="http://www.gravatar.com/avatar/00000000000000000000000000000000?d=http%3A%2F%2Fexample.com%2Fimages%2Favatar.jpg"/>
View the docs for more options and info.