See whether your screen can resolve all the dots in the userpic properly or whether you get “beats”, smushed areas, colour banding, or other artefacts :)
Incidentally, I’m a bit surprised that the GIF version of this image was nearly three times the size of the PNG version; my experience had been that for very small images, GIF can win. I guess that’s true only for sub-100-byte files where the size of the header starts to matter, compared to the compression ability.
Edit: saving the icon as a greyscale PNG instead of a palette one shaved another 1/6 off the filesize—now it’s under 100 bytes! (I’m a tad disappointed that the PNGOUT plugin didn’t do that for me in its “Auto” colour type setting.)