Determining a tween motion item's base item?

[NeoJigglypuff]

Here is the scenario:

I imported an image (SampleImage.bmp), treated it as graphic, and made a simple 2-keyframe tweened animation, thus my library contained the following items:

name type
SampleImage bitmap
Tween1: graphic
Tween2: graphic

I run through document -> timelines -> layers -> frames -> elements and try to enumerate the name of the elements, which would only contain only one object - the imported image. However, instead of using the instance name (elements.name), I want to use the library item instance through element's instanceObject subclass, so I do elements -> libraryItem to get the name (libraryItem.name). I got names "Tween1" and "Tween2" for each respective keyframe, which would be right. However, I am trying to figure out how can I get "SampleImage" name from Tween1 and 2 since the tweens are based on SampleImage. Would a library item get to refer to another library item? Or something like that.

Thanks!