If you have images flowing then you're in the right state. I agree that you should not try and control the images but I would always try to analyze them and build on them, make the images more vivid. I would concentrate really hard and I'd do it again and again no matter how many times the images dissipated. For me when the image is defined enough I'd look up or move through or just look around and then I'd be in a lucid dream. The things to focus on are up to you - working with whatever pops up is always good. I used to work lots with images of books and bookshelves or pages in a book - I could then focus on trying to read the words on the pages or on the spines of the books bringing more definition to the image. Dream scenes from previous dreams are also excellent to use.