Nothing in your code sets the _y property of the image so first you should tackle getting something working for that. Then see about using the bottom line you mention as the starting point from which all thumbs get placed.
myThumb_mc._y = _root.thumb_to_bottom = 20; // its not working
The _y property of the image designates the upper left corner of it, so you need to adjust the position based on the height of the thumb if you want to position the bottom at some specific value.
If the bottom is 20, then you want to set the _y property to be 20 - image._height...
myThumb_mc._y = 20 - myThumb_mc._height
its not align the image,, i want that when i upload a small images (200 X 300) , its start from top to bottm, but my requir is its start from bottom to top.,,,
its also not working,,
_root.myThumb_mc._y = Stage.height/2 - _root.myThumb_mc._height/2;
YOu need to think about whatever it is you tried to do with that line of code. You have failed to define where the bottom is so there is not much anyone can do to help you. You basically need to keep track of where the last image was placed by changing the bottom value each time you place an image and then add the next image using something more like the line I showed where you subtract the height of the new image from the top of the old image (which is the new bottom)
myThumb_mc._y = 210 - myThumb_mc._height
in this conditions the size of large image increasing top to bottom, but not bottom to top