Hiya,
Open the common.css file for your chosen gallery ( /gallaries/ribbon_lightbox/css/common.css) and at about line 11 of that stylesheet there is a rule called .pg-rl
Add a margin:auto line to it, i.e:
.pg-rl {
width: 896px;
font-family: Arial, Helvetica, sans-serif;
margin:auto;
}
If that doesn't do it, then post back with a link to your gallery page, and we can sort it out.