I am working on my page speed. And I am doing test

I am working on my page speed. And I am doing test on pingdom. And I seem to have a big problem with gravatars see the test : http://tools.pingdom.com/fpt/#!/8nvzO/http://bimv.pl/kurs/wyzwanie-z-revitem/
How can I disable permanently gravatar requests and use a default image instead. I can't find sollution in web.