Hi Paul,
merging two account is not possible.
Besides that - it seems Panoramio administrators team has left and forgot to switch off the light (only Pano admins technically can do that merge).
However, It is possible to find some useful hints about this issue here in FAQ, chapter 7:
More hints: To log in to Panoramio legacy account (not upgraded to Google Account), sign out, go to Panoramio sign in page and click drop down arrow beside "Sign" button and choose "Use a legacy account".