隐藏较小的图像部分，不与背景中较大的图像重叠

I have two images, one larger than the other. My need is to be able to drag and place the smaller image anywhere above the larger one. The problem is that, I don't want the portion of the smaller image outside the larger image's boundary to be seen. Once the smaller image's non-transparent portion stops overlapping the larger image non-transparent portion, the non-overlapping smaller image portion should be hidden.However, when the smaller image is completely inside the larger image's non-transparent portion, it should be visible.! I have two images, one larger than the other.