I've had this happen.  When it does, it turns out that Poser has renamed the texture node to Color Texture instead of Image Map.  Delete it, load a new Image Map node and assign the image to that and everything works then.

I don't know why Poser does this, but it happens very seldom.  Next time this happens go to the material room and see if the name of the node has changed to Color Texture.  Maybe someone knows a fix for this.

John