Scale is supposed to go off the image's original dimensions, so I do believe this is a bug with Tween.
If I set an image's size or scale to 0, the ImageWidth still reads as the original value (in my case it's 64) which is the correct behavior. If you set the Scale directly, it scales to the correct size, ie. setting scale to 0.5 make the image half size. But if you set the scale to 0 and then Tween the Scale from 0 to 1, the Scale stays at 0.
Looks like the Tween Scaling is multiplying against the Width and Height (current image size), instead of ImageWidth and ImageHeight (original image size). This has my game broken unfortunately. :(