Creating a Transparent Image with White Outline

[Isaac5C51]

 

I am looking to get some help on an effect that I would like to do.  I actually did this effect myself a couple of times in the past, but now I cannot seem to remember how I ever did it.  Basically, I took an image of my dog and created like a white transparent outline of him and it turned out really cool.  I attached the original picture and the end picture of what I did.  If anyone could help explain how I can do this again, that would be great - thanks!