I definitely cast, but like many have said, it has to be someone I don't know. There is no way I could cast famous people.
I have an idea in my head, say race/hair colour/age, and then I have to find a picture that looks similar to what I'm envisaging. This can take some time!
When I write, the characters act out the scene in my head (much like a film), so I need to be able to visualise them. I often seek out visual representations of clothing, locations etc etc.
I find unless I have a face and a name (I can spend days looking for the right name), I cannot get them to act in my head. Personalities, mannerisms and voices come from this acting, so that isn't picture specific.
I work on Scrivener, so I have a character template complete with photo for each of my main players and a few of the minor.
I couldn't work any other way.