I've had a few different ones since I joined. My first was clipped from an image of Astro used on the right stuff boxed set. I then made a few 'floating' images with the gimp from various posters done by Tokyo Denki University. My current one was part of an image that Astro Forever made for the photoshop contest thread. That one was from the 1980 series, and she colored Atoms hair grey. I sorta identified with that image and asked her if she could put a pair of eyeglasses on him.
The avatar I'm using on FanFiction.net was done for me by April7, and is literally "Tetsuwan Penguin", a rendition of 'Tux' the Linux Penguin with Astro hair spikes, red feet, and a green belt.

BTW it was a bit of a challenge to make that Avatar for Earthshine due to the file size limit. I had to play with settings in image compression, frame rate, and number of pixels. The first effort was a bit choppy until I figured out that the Gimp was throwing in a few invisible layers and I finally figured out how to remove them.
EDIT:
Shiyonasan:
I was looking at the thread in the link you posted. Now this image is interesting ...

It looks like the one in the manga story "The Angel of Vietnam" where Atom smashes into a B52 bomber and scares the crew into bailing out and aborting their bombing run.