I having a bear of a time trying to find the cause of the issue, if I strip your page down to just the power gallery, it will will work fine in Opera, I then restore it to the origianl, and it continues to work fine in opera.
at one point, I had striped out the following:
<br class="clearfloat" />
and it started to work, but it didn't break again when I put that back in. I am getting completely inconsistent results in trying to trouble shoot what is wrong with this page.
I am not able to reproduce the issue with opera in the Power Gallery demo page:
Where the demo page is working correctly, it leads me to believe there is some conflicting CSS on your page that is causing power gallery to break, but the inconsistency of my testing is making it impossible to pinpoint the exact cause.