FYI: GIMP is a free "photoshop-esque" program. It does all the basics that photoshop can do. I recommend it for those who don't want to drop the dough on photoshop. (Me)
First, open your image in GIMP. (File, open, choose your file)
Next, if you have chosen a color file, turn it into a black and white image by going to Image, then Mode, then Grayscale.
To get your outline, go to Filters, down to Edge-Detect, and over to Edge.
When the box pops up, click OK.
Now you have a black image with a white outline.
To get a white image with a black outline, go to Colors and down to Invert.