Well, for me imagining a tiger is basically recalling what it's like to look at a tiger (or picture of a tiger). I can have a photo of a tiger right in front of me, and know it's a tiger, yet not know how many stripes it has without counting. I might also not notice how the back paws look if I'm focusing on the front paws. Same thing when it's in my head (except less consistent, since it's just a memory).
> how do you know what position to put the stick figure in -- akimbo, Thinker?
This is no different from other senses. If I tell you to imagine the smell of soup, do you smell minestrone or pho? (Have you cooked those dishes? When you smell soup, can you tell what spices are in it?) If I tell you to imagine the sound of a violin, is it a six year old scratching away or Yehudi Menuhin playing a Beethoven concerto? (Depending on your musical training/listening habits, you may be able to imagine a concerto all the way through, or maybe just fragments of it?)
The interesting thing is, at one point I thought, like you, that I couldn't think visually. I tried practicing it, and either that worked or I adjusted my definition of visual thinking (or a bit of both), because I definitely consider myself at least partly a visual thinker now, and it's difficult to imagine it any other way. I've noticed that my visualization ability is sensitive to how much sleep I get though.
If you can think of chocolate, but not how it tastes, how it feels in your mouth, what a square looks like, what sound it makes when you bite into it ... maybe you just need to pay more attention the next time you eat chocolate :) And my ability to imagine other things - like music or food - depends strongly on how much time I spend on such things.