Set default user image

Hi everyone,
I’m a ruby on rails programmer and I want to improve my skills using forum and everything possible to get knowledge.
I have a problem: I cannot set up the default image of my users. How can I do?

Thank you everyone for the answers.

It’s simple! Just go in to your account preferences and set up your default image.

Thank you for the answer, but how can I find the function that create my account?

I cannot understand this