Try attaching it to the foot, and then changing the Z value of the item so it is in front of everything.
You can also try dragging the image into the vTuber Kit window in the latest version and that might set the Z value to display in front of the avatar by default, I can't remember 100%.
You could also spawn a new custom item and then set the z value to -500 and that should work. I included a gif as an example