It would be much easier to use a showimg and just clamp the x and y you display it at to 8 pixel increments (half tile) then when placed, actually use the putnpc2 to lay it.
NPC Code:
mx=mouse.x; // get mouse x coordinate
my=mouse.y; // get mouse y coordinate
mx-=mx%8; // clamp mx into 8 pixel increments by subtracting the remainder of a division by 8 (mx modulus 8)
my-=my%8; // clamp my into 8 pixel increments by subtracting the remainder of a division by 8 (my modulus 8)
// showimg goes here using mx and my for coordinates
but wait, triggers are bad! =( sorry lance.