Can you help me with this avatar problem? See detail?

I am trying have an avatar (the curret one is Fluther’s) in Fluther. Someone emailed me avatars. Am I supposed to turn it into a JPEG first? so I should open “My Documents? and then…(I don’t know what I am talking about, I’m asking to be corrected) If it is correct so far, can you continue for me? I need a step by step instruction. 1)...2)... Thank you in advance.
I can’t see, therefore refer to some of the answers that I got between signing out and signing in which is when the question was put away for editing)