public void setOverlayImage(int index, @Nullable Drawable drawable) {
    // Note that overlays are by definition top-most and therefore the last elements in the array.
    Preconditions.checkArgument(
        index >= 0 && OVERLAY_IMAGES_INDEX + index < mFadeDrawable.getNumberOfLayers(),
        "The given index does not correspond to an overlay image.");
    setChildDrawableAtIndex(OVERLAY_IMAGES_INDEX + index, drawable);
  }
Sets a new overlay image at the specified index.

<p>This method will throw if the given index is out of bounds.

@param drawable background image
drawee/src/main/java/com/facebook/drawee/generic/GenericDraweeHierarchy.java