Even though, it is quite easy to figure it out with a little PHP and Drupal knowledge, it might still be frustrating for beginners to find an easy answer for this question.
Try creating a block with the following PHP code (do not forget to select the PHP code as input format):
$owner = user_load(arg(1));
print theme('user_picture', $owner);
Most probably, you will want to limit the visibility of the block to
users/* (if you are using pathauto module).
Source: dalad’s comment