best method so far:

1. get rid of jpeg artifacts with selective gaussian blur, blur radius 2, max. delta 15
2. image scale with lanczos3 from 118x118 to 700x700 for example

the image is a bit blurry after that, but smooth and not chunky/aliased

apparently lanczos3 can introduce new jpeg-artifact-like ringing, or pixellation. This can be fixed using another selective gaussian blur but with proportionally bigger radius.

todo: implement this using perl's Gimp module!  done :)