Aside from the architecture, sculpture and famous paintings, there are several other types of "artwork" to be sampled. Some of this "artwork" takes the form of posters and stickers, there are various types of graffiti throughout La Serenissima too. Another method some individuals have used to mark their territory is through the use of stencils.
A stencil is something with cutouts which can be used more than once to make the same impression - often using sprayed paint. Seeing as how it's against the law to mark territory in such a manner, the stencil gives the painter the ability to not only make the same mark several times, but to do it quickly in hopes of not getting caught.
Here's a small stencil I saw in Dorsoduro.
It appears to be a red cat.
I'm not sure of the significance behind it, but it probably carries meaning for the person with the spray-can.
There appears to be something in the cat's mouth, but I can't quite make it out.