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I'll probably change my current one when I think of/create/find a better image, but for now I use one of my 'illegal' (long story) artworks, a shiny recolour of mine, of the official art for Giratina's Origin Forme, since I have 2 shiny ones and nobody else had recoloured the official art to be shiny. Here's the current avatar in full size:
I love Hundertwasser's art. I think he does his trees nicely. If you've never heard of him, go and have a look. He did all sort of great images and architecture.
Easy for me: I love Georgia Tech (and by extension, absolutely fucking hate UGA). I googled "Georgia Tech avatar" and this fit the size limits, so tada. I might change it to something Pokémon-related down the road, though.
Mine represents the awesomeness of space and the miracle of it. I also really like astrophysics and it was better than my last avatar which was the worlds ugliest dog.