I think I have found the problem. My soft links are all broken! In the Calculus_I_NYA_MC directory, the gif files are pointing to Calculus_I_NYA/templates while in the Calculus_II_NYB_MC directory, the gif files are pointing to Calculus_II_NYB_MC/templates
The "MC" was removed from the Calculus I path. I have no idea how this happened but at least I can fix it.
Just for completeness, I really do not have any .htaccess file.
Thank you for your time and help