Not sure exactly why, but viewing the source of your page the javascript to launch the large view images matches ours:
//Show the large image popup
function ShowLargeViewPopup() {
$find('ctl00_cntMain_UC_ProductView_tbcProduct_tabMain_fvwProduct_UC_PopUpLargeView_popMessage').show();
}
However, when I search for this control, it appears the javascript that .net writes to display this is slightly different in your page.
I think the following will fix it... if you open up UserControls/Front/ProductView.aspx and find the function above there. Then change
ctl00_cntMain_UC_ProductView_tbcProduct_tabMain_fvwProduct_UC_PopUpLargeView_popMessage
to
cntMain_UC_ProductView_tbcProduct_tabMain_fvwProduct_UC_PopUpLargeView_popMessage
I think also should need to remove the ct100_ part from the media function immediately below that too.
Not sure why your controls are differently named, I believe this part of the name comes from the master page, with ct100 the default if nothing else set.