Oops! Sorry guys, I was wrong. There's not another Annunciation but there is an Assunta! I messed up when translating one of my books but found another (in English) that identified this relief as an Assumption scene.
There are two shrines on the Rialto bridge - one on the San Polo side and this one on the San Marco side. Mary is being lifted up to Heaven by the crowd of cherubs below.
The wine bar next to this shrine is a fun place to sit and watch all the traffic on the Grand Canal. A photo of this bar was posted all over the Internet after the especially high acqua alta of December 2008. My photo shows it dry; click through for the wet view.