Here is a topic with lots of links (also view the other posts in the topic as not everything is updated in the first post):
http://forums.sketchucation.com/viewtopic.php?f=40&t=486
When you import an image and scale it properly so that it's at a correct size, it will be part of your model. Now you can use this plugin to quickly change a material applied on your model with another one in your "In model" library (obviously the one you have just imported).
http://forums.sketchucation.com/viewtopic.php?t=9903
This is another one:
http://forums.sketchucation.com/viewtopic.php?t=26013