The BBS software only contains an image link to the URL you specify in your settings. It puts such a link in for each post. If you change your avatar link, the new setting is not retroactively applied to your old posts.

The two different paradigms are:

• You change the URL in your settings
  All your old posts point to the old avatar; all your new posts point to the new one
  
  
• You change the image located where your URL points
  The images on all your posts, old and new, change.