I am replying to your question of what I see when I image stream. I am surprised by the number of people who seem to see very little.
I assure you that image streaming really works, in that I really see extremely engaging things when I image stream. It is closer to having a very controlled lucid dream than anything else I have experienced.
As for all of the IQ benefits posited by Win's book, I don't know. I do know that it has started to become an invaluable source of creative inspiration from which to draw. It is also EXTREMELY entertaining.
I have some adaptations to the basic methods discussed in the book. I don't use a tape recorder, nor do I speak to another person. I speak aloud to myself, sometimes I whisper to myself when I think I can be overheard. I also find it good to cover my eyes with my hands sometimes to block out ambient light.
If all you can see is splotches of light when you start, then describe them in as much detail as you can. Eventually, you may realize that you are having difficulty concentrating on that description because another image is trying to force it's way into your conscious mind. This is what you are looking for... this new image is the one coming from your subconscious... just describe it and you are on your way to image streaming.
Hope this helps.