How do i take a photo from the internet and make it into an avatar on another website?

I am pretty un-technical and I would like to find a photo from the internet (or maybe use a photo that I have taken) and make it into an avatar for myself on another website. Please give basic advice to a technically-challenged person.

Just FYI, if you’re wondering why I’m asking this since I have a Fluther avatar, another Flutherite did it for me. He is someone I trust and I gave him my password and he did it for me. However, I would like to learn to do it myself.