There is a ThumbArea.as file you might try to tweak, but in this case, you might be better off editing Options.as, setting line 17 to true (to set static layout).
static var fixedLayout:Boolean = true;
And then setting the x- and y-coordinates of the thumbnail block in lines 84 and 85 of Options.as:
static var thumbAreaX:Number = 500; //X posn of Thumb Area static var thumbAreaY:Number = 10; //Y posn of Thumb Area
to whatever values work best for your layout.
I found that it was a lot like designing a website: try a value, save, republish your .FLA file and preview the result, change again, preview again, etc. until you arrive at pixel-precision!