Hi,
I have come across similar issues with the AlexC stylesheet, which I think you are also using. It doesn't seem to reliably centre the thumbnails (at least in Internet Explorer). This isn't noticeable if you use the default settings (4 columns, table width 75%), but appears if you create extra space around the thumbnails (by increasing the width or having fewer columns).
I have found that removing the line
width: 200px
from the stylesheet seems to fix the problem, although I have no idea why. This is the approach I have used for this gallery:
http://www.pbase.com/alangrant/oldscans
I am a little worried that this line might have been included in the first place to prevent some other problem, so if anyone happens to read this and notices anything odd about the layout of my gallery, please let me know!
If you want to try changing the stylesheet as I have described, there are instructions at
http://www.srijith.net/pbasefaq/ in he HTML/CSS section.
I suppose the other options are to play around with the gallery settings, for example increasing the cell padding or having 4 columns, or to simply use a different stylesheet.
Hope this is of some help
Alan