#1) Start by choosing the image you want to use. Look for one with a good profile and clean lines; color and pattern don't matter, but do make sure it has a transparent back. Here is the one I'll be using for this tute.
#2) When you've decided on an image, the first thing you need to do is convert it to solid white. The easiest way I've found to do this is to take the image to GifWorks and choose "Effects" --> "Monochrome". This will convert your image to black and white. Then choose "Edit" --> "Replace Color" and replace the black in the image with white. You will now have your solid white image.
Save this image to your own scrapbook or other account in whatever manner you're accustomed to.
#3) Once you've saved it, take the url back to GifWorks and reduce the image's size, being sure to keep the checkmark in the "constrain proportions" box. The smaller you make it, the thicker your finished outline will be. Next, choose "Edit" --> "Replace Color" and change it to solid black. Now choose "File" --> "Save" and copy the temporary url to your cache - you don't need to save this image permanently.
#4) With the temp. url copied in your cache, go to Image Magick, type in the url for the solid white image you made and saved in Step #2, and press "View". Click on the "Composite" tab. Now paste the temp url from your cache into the box. Use center gravity and choose the "Minus" option from the drop-down menu. Leave everything else the same and click "Composite". The resulting outline shape will be black and white. Output the image as a gif and copy the url to your cache.
#5) With the temp url copied, go back to GifWorks. Choose "Edit" --> "Replace Color" and convert the black to white. The result is your white outline.
Save the image and you're done. Now you have both a solid white image and an outline image to play with!