Personally, I think you should pick one high-quality avatar that really embodies who you are, and stick with it. Most forumers recognize a person by their avatar so constantly changing it won't really help.
For example, I chose Toothless the dragon because I have a more laid back, fun-loving and goofy personality, just like Toothless, but can also be super serious when a situation requires it. I'm also a furry and a huge fan of the how to train your dragon series in general, so it's perfect for me.