( you need some sort of search function in your forum - then I may have found something there)

I want to understand how the avatars are generated -  where exactly are they? how are they named? how can I upload one? I see the backend directory option - which says where are your avatar files - but I put them there ( and in any case had to create the directory??)  idnumber.jpg and they aren't recognized - so how does this work?
So
1. How are profile/avatar images named
2. where is the code that fetches the images to dispay

Thanks in advance