Hello Keith,
You'll need to add the colorbox css to your skin. Older versions of mojoPortal (prior to 2.3.6.4) used GreyBox in the image gallery, so your skin is probably setup for that.
Anytime you're upgrading, you should check the "Important Skin Changes" document to see if there are any skin changes associated with the releases between the version you're running and the version to which you're upgrading.
HTH,
Joe D.