Setup custom default avatar

Hi, I would like to setup a custom default avatar for users that do not have any pic associated in gravatar.com

I checked the following site:

http://codex.wordpress.org/How_to_Use_Gravatars_in_WordPress

But I wonder if the change will be overwritten whenever you release a new version of the theme? If yes, would there be a way to make it permanent?

Or is there actually an easier way to do this and also make it permanent?

Thank you for your kind support!