This painting is called The Young Martyr, or the Young Christian Martyr, or A Christian Martyr Drowned in the Tiber during the rein of Diocletian, painted by Paul Delaroche in 1855.
Delaroche was a French painter who had a great liking for painting historical motives, for example The execution of lady Jane Grey and The princes in the Tower (he seemed to have a special liking for English motives, despite his nationality). This portrait of young, dead woman is not English, though, but set in Rome, 1700 years ago.
The dark silhouette of a man can be seen against the darkening sky and the sunset, but the main focus is on the dead, young girl. Her hands are bound and her eyes closed - and her features are lit by a halo over the face. She is dressed in light fabrics, totally soaked but still floating.
The persecution of Christians during the rein of Diocletian (reining 284-305) was all to real, and at their height at 303-304 - a time that would bring forth a great set of saints, people executed due to their faith. But even though he was a determind man the persecution was not a great success - Christianity survived after all, without a great slump of numbers of members - and even a great number of non-Christians were against the persecution. The laws prohibiting the Christians to live out their faith would last for another 25 years, but they were not acted on for the most part.
The motif is walking the fine line between being creepy and overly sweet - but perhaps more on the eerie side of things when you take into account that the woman has the same face as Delaroch's wife. She had died in 1845.
The painting can now be found in the Louvre, France.