Some user images switched after upgrade
Drupal 6 did not have {file_managed} at all. Also, user profiles wasn't really fieldable. And both styles and fields changed between 6 and 7. 3 big changes affecting one value - that's bound to create problems.

To get around it, you can use Views Data Export to expose users from old site, and then import your users back using for example Feeds


  Import or aggregate data as nodes, users, taxonomy terms or simple database records.


with Feeds Image Grabber


  an addon for Feeds module, which grabs image for each feed-item from their respective webpages and maps it to an image field