It should roughly look like this:

import poser

scene = poser.Scene ()

use the currently selected actor

actor = scene.CurrentActor ()

use the first material that is there:

material = actor.Materials ()[0]

manipulate the shader tree...

shader = material.ShaderTree ()

create a new image map

image_map = shader.CreateNode (poser.kNodeTypeCodeIMAGEMAP)

set the path of the image_map

path_input = image_map.InputByInternalName ("Image_Source")
path_input.SetString ("d:my_image.png")

connect the nodes

surface = shader.NodeByInternalName ("PoserSurface")
shader.AttachTreeNodes (surface, "Diffuse_Color", image_map)