It's the problem with the textures ... if you compare the textures between 1.3 and 1.4 updates - in paint, you will see big differences in the borders of the textures of zombies. All you have to do is: take the zombie photo from "appdata/roaming/tekkit/texturepacks/Sphax 64x Tekkit SMP Latest v1.5/mob", shrink the bottom border to the lowest pixel of the zombie picture (in witch the zombie it self is showed) and it's done... Thought I don't know a program witch would so easily manipulate the picture like that and wouldn't leave the white colour instead of transparent...But you could just do it in paint and then, after finishing the fix, use mcpatcher3D in order to make it transparent... anyway you will only have to work extra on the scull zombie, cause you can't copy it from the older version.