What are your interests? For example, if it's movies, animes, video games, etc., you could choose a picture of a character. If there's a musical group you like, you can choose a picture of the group or some artwork from one of their albums. (These two examples are basically all I've ever done.)
Ultimately, your avatar is an expression of you, and so while I understand why you might have a hard time finding one, I don't know how well anyone else can come up with an idea for you.
Edit: Speaking of Chew's avatar, I know I always thought that it was the head of some thin snake-like thing. I was shocked when I found the original image:
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