The paradigm shift that I had was that practice makes habit. (I'm currently writing a blog entry about it, but it won't appear for a few more days, at least, not before the weekend.)
If you practice visualizing dark images, then you're more likely to use the dark images in the future... If you practice forcing yourself to turn up the brightness, then the more you practice, the easier it will be to have bright images.
One way to practice is to have a bright light in front of you (keep your eyes closed
) and practice your visualizations that way. Even though your mental imagery and what you actually see are seperate, the brightness shining through your eye lids will still boost the brightness of the visualization.