Hit effect on image

Ok so basically im trying to create a hit effect after a thumbnail has been clicked or shown.
My thumbnails are located here:

_level0.mc_main.menu_tot.menu.pic13

with pic13 being the thumbnail. Doing this works fine:

_level0.mc_main.menu_tot.menu.pic13._alpha = 50;

but doing this doesnt work:

var increase = pic13;
_level0.mc_main.menu_tot.menu.increase._alpha = 50;

why doesnt it work? Is there a way around this?
Peace.