EL17 - Sorry for the late reply. I used to make up my own stories for my son while he closed his eyes and imagined the images to my story. It could be at bedtime or just some quiet time during the day. As far as I recall, the important part of imaging is to engage all the senses. Don't just talk about things your child should see, but what you can taste, smell, feel and hear.
For example...
Today, we are sitting in a forest. There is a waterfall nearby and the water is crashing down upon the rocks with a sound like thunder. The air smells crisp and clear. We go over to the water and taste the icy cold mountain water.... and so on.
Another thing you can do, if you don't want to come up with your own scenarios is to play audio stories. Learn Out Loud has a lot you can download - some are free.
http://kids.learnoutloud.com/I hope that helps!