Are you saying you import the images in psd format? Well anyway, I would suggest to explode the image if you are importing as an image. SketchUp will create a face (true geometry with the four edges enclosing it) which you can turn into a face-me component quite easily. Below is a short video how to do it (it's a Roman soldier but trees work the same way) http://www.youtube.com/watch?v=VvI6A7Njw4k