I tend to be stubborn when things aren't like I want. So like you I wanted my thumbnails centered on the page. I've been doing sites since way, way back in the day when css didn't work the same in any of the browsers, and I had to do a lot of "cheating"... but hey, it worked and if the "big guys couldn't get their acts together, I made things work in all the browsers the way I wanted on my end.
So, I spent some time tinkering and tweaking... and finally I got my thumbs centered... somewhat of a "cheat" but my clients don't care if it looks good... It has to do with how I placed the [nggallery id=XX] code on the individual page using the visual editor.
First I tried a centered div tag. <div align="center" style="text-align: center;">[nggallery id=XX]</div> NO GO... didn't work.
Then I tried placing the tag in a table with the td centered. NO Go... didn't work.
Tried a bunch of other things and then I found it! First I uses a simple centered div tag. Next I place a single cell table at 90% inside my div tag. Then I placed the short code inside the single table cell. WORKED!... in IE, firefox, Google Chrome and safari.
There is the exact coding I pasted in the visual editor:
<table border="0" cellspacing="0" cellpadding="0" style="width: 90%; padding-top: 0px; padding-right: 0px; padding-left: 0px; padding-bottom: 0px; border-top: 0px; border-right: 0px; border-bottom: 0px; border-left: 0px" align="center" width="90%">
<td align="center">[nggallery id=10]</td>
Here is the page: http://www.fvapollo.com/fvapollo/gallery/
Hope this helps.