I have previously written above visual representations of iconoclasm (have a look here). Arguably, these can be interpreted as part of a “visual polemic” (Elsner’s term) against paganism. A fresco in the Dura Europus synagogue could be interpreted in the same way, but it is, of course, a depiction of a biblical scene. Wikipedia has a good article on this story from the first Book of Samuel.
Depiction of the fallen idol Dagon in the Dura Europus Synagoue, Syria. Dated c. 245 CE. From Elsner 1998, p. 216.
Jas Elsner. 1998. Imperial Rome and Christian Triumph: The Art of the Roman Empire, AD 100-450. Oxford.
Carl H. Kraeling. 1956. The Excavations of Dura-Europos. Final Report 8.1. The Synagogue. New Haven & London.
Online stuff on Dura Europus.
Simon James at Leicester maintains a Dura Europus page with a good bibliography and a short history of research.
All the frescoes in the Synagogue can be seen on EIKON.
Also check the Aarhus Department of Classical Archaeology’s slide collection for views of Dura Europus – although not all of the slides have been scanned yet.