That's because your selected board style is prosilver. You can change that in your User Control Panel, Board Preferences tab.
Prosilver is the default style for PHPBB. Out of the box, it places the User Information panel on the right side of posts. Prosilver Special Edition inherits it's properties from prosilver, thereby functions similarly. I thought about modifying prosilver to display avatars on the left side, but decided to leave it as it is.
All other styles were modified to display the User Information Panel on the left side, in keeping with the tradition of the now defunct rsred style that was used prior to the implementation of PHPBB 3.1.10.