The Shi'a have a well-developed tradition of religious iconography. Their use of various images for religious motifs is not unlike that of Catholics or Orthodox Christians, although the other-worldly qualities of the faces of the figures depicted is more reminiscent of Orthodox icons than paintings by the Flemish masters. The most prominent figures portrayed are Imams Ali and Hussein, the latter of whom was martyred and became the central figure of Shi'ite history. During the protests, these signs, as well as the flags with Ali or Hussein emblazoned on them, were pretty much everywhere. Since March, however, many of them have been removed, and you are only likely to see them in smaller towns.
During the protests and, to a lesser extent, since the crackdown, youth have vented their anger through graffiti. Much of the graffiti is painted over, but in some towns it remains up, or is put up again. If you can read Arabic, it is interesting to see just how much the Bahraini youth have been affected by the protests in other countries - likely thanks to Aljazeera and other satellite channels. If you're looking to photograph graffiti, be discrete, as the authorities are not to fond of people documenting opposition to the régime.