The alignment of the table that contains the thumbnails cannot be changed with html files from the template but has to be changed in the php files.
True for vertical alignment but not required for horizontal alignment. You can achieve horizontal centering by editing thumbnail_bit.html This is a better approach, in my opinion, because you won't have to restore your edit with a new release of 4images.
Simply make the following line the 1st line in the template:
<div align="center">
And close the html tag by making this the last line in the template:
</div>