In general, I don't picture people in real life when I read the books... except for Fred and George Weasley... a guy in my youth group when I was in high school had red hair and he was the same kind of personality as F-n-G... Very funny, loved getting into trouble, loved making other people's lives harder, but you always forgave him of it because he was so charmingly annoying.
When I read the books, I immediately pictured Maggie Smith playing McGonagall before ever seeing the movie. I agree with what Rowling said about Daniel Radcliffe totally looking the part. As much as I do NOT think he's a great actor (though, I do think he's improving... I mean, how much can you hold him accountable for not being incredibly talented... he was virtually unknown and 11-years-old in P/SS), no one else will ever look the part as much as him, I think.
I kinda picture my sister when she was in about the sixth grade when I read about Hermione, or at least I did until I got so used to Emma Watson being Hermione... at that age, my sister had very thick, course light brown hair and she needed braces, because her upper teeth seemed very large...
In general, though, I picture actors (even if it's not the actors from the actual films) when I'm reading. When I read Lupin, I still see him as Ralph Fiennes, not David Thewlis. When I read Mundungus Fletcher, I see Rik Mayall (I think that's how it's spelled... he was King Herod in the most recent Jesus Christ Superstar)