webadmin: image column header - fix alignment

Amend style of image column header:
affects all headers that contain an image:
e.g. 'comment' column.

Before-Screenshot: http://i.imgur.com/Zhu25qB.png
After-Screenshot: http://i.imgur.com/4t7ntHe.png

Change-Id: I3bf543e6c1ec6a316da8a69520cc75878a68997a
Signed-off-by: Daniel Erez <derez@redhat.com>