The Rialto Bridge, built in 1524 but preceded by wooden bridges and before that by pontoons, was the only bridge to cross the Grand Canal until 1854. The covered ramps carry small shops on either side of the bridge.
The graffiti are more recent, I believe.
In the past 150 years, a few more bridges crossing the Grand Canal have been built, but you still have to walk quite a while to get to the next one: Accademia Bridge.
But there are more of the little bridges in each of the quarters, leading you on labyrinthine paths which always seem to lead deeper into the city, and never out of it.
The Bridge of Sighs (Ponte dei Sospiri) is not called like that for romantic reasons, but because it is the last bridge that convicts pass(ed) on the way to prison.