I also had this problem and could get nowhere with the css to center them, so I have come up with a very simple solution, although it is less than ideal – but it saves messing around.
Here’s how to do it:
My post area is 900 px wide.
So, in the post where I want to add a gallery I create a table 900px wide, with 3 columns, and an invisible border (ie border=0).
I then put the gallery in the center column.
The 2 outer columns automatically resize themselves equally, and the result is an image gallery that is centered on the page.
Like I say, less than ideal, but quick and easy.
I hope it helps someone.
