Hi Theerapun,
1. I don't have strong visual images either but that doesn't prevent me from doing it. My visualizations are "ghost-like" images. Sort of like outlines drawn on glass. However, it does improve as detail is added to what I'm saying. If I don't see detail, I make it up. To be honest, I've gotten more vivid images while listening to paraliminals. I'm not image streaming then but I definitely get much clearer images while that relaxed. Perhaps you could start off with a paraliminal if you have one. Another good way to relax is to use the Silva method relaxation technique. Or you might find a relaxation MP3 on the web. Win talks about the Squelcher and that seems very valid. Real relaxation seems to get the Squelcher out of the way.
Have you tried any of the other techniques like "Old Dream Recall," "Afterimage Method," "Story Method," or "Fantasia Method?"
For beautiful images, you might check the Lord of the Rings movie site. It seems to me there were some beautiful images captured from the movie.
2. Yep, it makes you feel kind of self conscious talking out loud to a machine. Would it help to imagine one of your good friends was in the room and you were describing it to him or her? Would that change the details? It doesn't matter how "good" your descriptions are. You'll get better over time. You aren't going for an award here. The point is to go for detail.
Don't say the tree is green. Explain what the leaves and the bark look like. Can you recall a time when you heard wind blowing through leaves? If so, describe that too. What kind of sound has that made in the past?
3. Throw in other senses whenever it feels right to you. If an idea pops into your head about another sense that you could describe at that point, go for it. Don't strain to come up with something. If you can't imagine the sound of sticks cracking under your feet as you walk, just describe what you see. Just like PhotoReading, it's an acquired skill.
Are you trying to "see" the images in front of your physical eyes (eyes closed)? I did that for a long time and the results weren't good. I noticed that when I got vivid images during a paraliminal, they were behind my eyes. Sounds odd, but I think I was actually "looking" in the wrong place. I expected the images to be almost out in front of me, like a TV or movie screen.