I tried to follow the info here by editing the css:
http://forum.coppermine-gallery.net/index.php?topic=20473.0
but it did not have the desired effect. Is there another way? I use the classic theme and now descriptions are center aligned. see here http://www.translatum.gr/gallery/displayimage.php?album=12&pos=2
Edit: I found it
Go to includes/theme.inc.php
and change the center to left
<!-- BEGIN caption -->
<tr>
<td class="tableb"><center>
{CAPTION}
</center></td>
</tr>
<!-- END caption -->
don't ever edit include/themes.inc.php, under no circumstances. Edit themes/yourtheme/theme.php instead. If a function or definition doesn't exist in your theme.php, copy the section from the sample theme first.