Use imagehoster properly

There multiple similar issues related to bad use (or not at all) of imagehoster:

• Previously mentioned #40 (closed)
• Don't access external images directly, always via imagehoster (potentially some exceptions like youtube video previews)
• Originally if there were avatar/small use avatar/small, not medium (default), it makes 4x difference in required bandwidth for no reason
  For example avatars in list of posts are within img that's 40x40, and 128x128 avatar is loaded instead of avatar/small (which is more than enough: 64x64)