I've been using Internet Explorer 8 for a while with no problem. Just tested it now with IE7 and got a similar, but not identical message:
Error: Invalid argument.
URL: [My website-removed for privacy]/photos/showphoto.php?photo=1644&title=1644&cat=1763
So, IE7 references "photos/showphoto.php" and IE8 references "forums/clientscript/vbulletin_menu.js
Aside from upgrading PhotoPost Pro from 6.23 to 7.0, the only other thing that changed in my environment since I last used photo options (without issue) was going from VBulletin 3.8.4 PL1 to 3.8.4 PL2. I can't check right now to see if "forums/clientscript/vbulletin_menu.js
" changed with that, but I don't think it did. Sorry I don't know enough PHP or JS
to be able to dig into this myself. I really do appreciate your help.