If only I was as good at spelling and typing as I am at puzzles.
If none of the Inscriptions were true, then the Gold Inscription would be false, meaning the portrait would be not be in the Gold Casket.
If none of the Inscriptions were true, the Lead Inscription would be false, meaning the portrait would be in the Gold Casket.
Since both statements cannot be true, then one of the inscriptions is true.
If the Gold Inscription was true, the Silver Inscription would be false.
Since the portrait can not be in both caskets, it is not Gold.
If the Silver Inscription were true, then the Gold Inscription would be false, meaning the portrait could not be in the Gold Casket.
If Silver Inscription were true, the Lead Inscription would be false, meaning that the portrait would be in the the Gold Casket.
Since these two statements contradict each other, it cannot be true.
If the Lead Inscription were true, then Gold Inscription would be false, Meaning the portrait would not be in the Gold Casket.
If Lead were true, then the Silver Inscription would be false, meaning the portrait would be in the Silver Casket.
Therefore, it is in the Silver Casket