"While you are doing Asana, do you usually choose one posture and keep meditating on that or do you switch postures at regular time intervals? e.g. 10 minutes in God posture, 10 minutes in Ibis etc. "

For a given practice session, one sticks with a single posture.

In the larger picture, it's common (probably routine) for people to try many different postures in order to find the one they want to settle on as their standard. (I'm speaking of A.'.A.'.'s primary use of asana. I'm not speaking of other broad Hatha Yoga programs where, in the course of an hour or so, one would work through many different postures for physical toning etc.) Even in this "trying several on for size" stage, it seems best to persist with a given posture certainly during a given practice session, and perhaps for several days to see how one "settles in " to it.

The goal, though, is to find a single posture that one can use for long periods of meditation and other work (or a very few postures, since some of them serve different purposes, e.g., a primary one for sitting, one for standing, etc.).

"And apart from the illustrations in Equinox vol I., is there any clear pictures of the postures? I try different positions every once in a while and I can not even stay in balance in Thunderbolt posture. Are there any tips and tricks for doing it properly?"

Practice, practice, practice <g>.

You can Google it. There are lots of Yoga sites discussing such things. That one is tougher than others, and very good balance, physical poise, and physical strength help a lot.

Maybe you should pick an easier posture to work with.