Pose landmarker input size problem

[JeJe-LIAO]

The Model of input size is 256 x 256 x 3 (shown in spec), but BitmapImageBuilder can can accept a bitmap of any size.
How dose it work ? (Change input bitmap size to 256 * 256)
Should I:

1. Fill the black edges to make the bitmap square, preventing distortion of the person
2. reszie to size 256

Thanks