uploading profile pictures?

i want to know that how to activate that option where every user could upload his own avatar/profile pictue?
i know i can do it for every user manually, but how can they do it for themselves manually and also can change it....

For every user,
Profile>Avatar

i didnt get this man.... how to do it, please explain

On your navigation bar there is a link called 'Profile'. Tell your users to click on that, then slightly below that navigation bar on the profile page will be the Profile section's sub-links. On those links, your users need to click 'Avatar' and on that page, they can choose what Avatar they want to use. That is what was meant by Avatar>Profile.

and what about uploading a picture from there PC? can they do that ?

Yes by following the above steps. You will see the option Browse click that and find your pic and click on Open>>Save