Most people dig around on the Internet for images they like and size them down to fit the constraints. Other people are arty and make their own. It all depends on what you like and your particular talents. For example, mine is a screenshot I took from one of my favorite films made to fit.
People do a search for them. They do a search on something they like and find a picture they like and use it as their avatar. Some use their own pictures of themselves and resize it for their avatar, some make a picture through some computer program and save it and also resize it and use it as their avatar. Some people even use a picture of their kid as their own avatar.
I normally just go to google, decide on a theme and type in "theme avatar" under images and see what comes up, go to 'see fullsize image' save it, and then upload it. If you need anymore help then just post here (and someone else will answer )
I beleave a concistant avatar is important, however it took me over a year to spot the listructional pic on the X-Plus package and use it here. Now I've seen it up here I think some past user had it and I'm not so happy with it.