Change Prologue default Gravatar

Hi,

I just install WP 2.5.1 and Prologue 1.3 on my website and I am having trouble trying to change the default (Blue G) gravatar to a customized one. I found several sites discussing coding for themes and gravatars but nothing specific for Prologue. And, since Prologue is a little different from the average theme, and since 2.5.1 already uses Gravatars, the suggestions don’t seem to fit the need.

Does anyone know a simple way to do this?

Thanks in advance!