Or painted by an old master as the face of an angel.

虽然前后都是句号，这仍然不是完整的句子，而是在painted 后面承前省略了成分: it was。

完整的句子应该是：

Or it was painted by an old master as the face of an angel.

至此，这是一个被动句，be painted as “被画成......”。

全句的意思是：或者它被一位老大师画成天使的脸。