My old avatar was my Mii playing a miniature Funhouse model I have, which I took using the AR Camera feature on a 3DS. In my current avatar, I used Miitomo to take a picture of my Mii playing my Stargate. I was really happy with being able to make a nice, hi-res version of my original idea! Overall it pretty much just represents my two biggest hobbies, pinball and (mostly Nintendo) gaming.
Edit: Woah, I just realized that today marks exactly three years for me on Pinside!