I noticed this the other day too, basically what's happening is that WPBook is overwriting the WordPress default avatar. I noticed this with my blog too just haven't found the time to fix it or find a solution.
For now you want to set the default Gravatar in WPBook admin settings to your 32x32 icon. I set the icons to 50x50 because that's what the Facebook square icons are.
If you want to disable the Facebook profile icons all together you can delete or comment out line 1195 in wpbook.php
add_filter( 'get_avatar','wpbook_get_facebook_avatar', 1, 3 ); the line that overrides the standard WordPress Gravatar with the WPBook one.
You can also modify how Gravatars appear by modifying the
wpbook_get_facebook_avatar function 1145-1173 of wpbook.php this is where you can set the size to 32 keeping in mind that avatars from FB will be larger.
Hope that helps if you need/want more information just let me know.
PS: aftever WP 2.6 (I think) gravatars are build in so you don't need the gravatar plugin.