Yes, your doing it right, not matter how you perceive your image stream is right for you. You may naturally be more aware of physical sensations and get an idea of what the 'picture' is as result. You might even hear it in the sens that you are making it up. Some of the imagestream 'pictures' seem like memory of things you've seen somwhere before. They are all OK. The idea is to keep going with what you perceive and keep describing.
The same goes when you image stream you book. Take whatever body mind signals you have and keep describing from there out loud is definately a must. When using this technique on a book it pays to remember not all images will be a literal interpertation of the book since the mind speaks in metaphors. So don't judge the information prematurely. Ask yourself questions before you start image streaming so that your mind has an idea where to go. Anything unusal that catches your attention ask yourself why is it there.
Freenoting has much the same concepts... let it flow and make judgements after you have finished. If you use freenoting remember that you'll need to spend 15 minutes at least writing to get to most of it out. Hang in there rather than giving up too soon.
Alex