Church of Santa Maria Antiqua, Rome, Italy
The fresco is identified as St. Barbara by the photographer, Prof. Steven Zucker. It is also mentioned in Farmer's article on Barbara, which dates it as 8th century.
Although situated in Rome, Santa Maria Antiqua was a Greek church. Most of its images follow the Greek practice of identifying the saint with an inscription. No inscription is visible in this photograph, but perhaps there is one that was just too faint to register. The only attribute is a small cross in the saint's right hand, the usual Orthodox indication of martyrdom.
More of St. Barbara
Source: Steven Zucker via Flickr Mind Hive