I was seeing the same issue with my theme based on zenpage but realised I was putting the `photo:true` in the wrong part of the colorbox code. It needs to be in the `a.thickbox` section like this:
`
// <!-- <![CDATA[
$(document).ready(function(){
$(".colorbox").colorbox({
inline:true,
href:"#imagemetadata",
close: '<?php echo gettext("close"); ?>'
});
$("a.thickbox").colorbox({
photo:true,
maxWidth:"98%",
maxHeight:"98%",
close: '<?php echo gettext("close"); ?>'
});
});
// ]]> -->
`
This will now correctly display a full image when it has not been cached.