I hear characters speaking. I think of my creative mind as being like my parents’ old huge television set where the sound came in first, followed by the pictures, you had to give it time to warm up before getting those pictures and sometimes you needed to give the box a clout on the side to make sure the pictures did come through. To date I have not yet had to whack myself around the head to force through my “internal vertical hold” but I suspect that may only be a matter of time!
Having said that, I think hearing voices is great for dialogue writing and you can get a good idea (though not always the right idea) of how someone looks from how they sound. Most of the time you’ll be on the right lines but there will be an exception to the rule that stumps you from time to time. Seeing pictures I suppose could give pointers as to how someone is likely to speak but doesn’t allow for people who’ve “bettered” themselves (or are putting on a false front).
I think finding the characters' voice is the most important thing and for me hearing the voices is more important than instantly being able to see in your head what they look like. All of my characters are an amalgam and I find knowing how they'll speak, what their general attitude to life is shapes how I write about them far more than their appearance. Having said that, with my rebel fairy godmother I knew she was going to be stubborn and brave. I used those traits to work out she wouldn't wear what was expected traditionally for someone of her ilk so then deduced what she would wear and from there it was easy to "see" how she looked.