Here is the picture I'm going to use:
First remove the background from the image. If you don't the next few steps don't work as well. I won't go into how to remove the background, everyone seems to have there own way to do it.
Desaturate to make the image black and white.
Adjust Contrast & Brightness. Here I've set Brightness to -80 and Contrast to 100.
You'll have to play around with these levels to get the right amount of black and white to create a good image. Remember that if you plan to actually carve chunks of the pumpkin out (instead of just scraping the outter layer of skin off) that you must leave bridges between the larger pieces and smaller peices. you can't leave bits floating about other wise you'll end up cutting off a bit of the nose or eye. Think of the inbetween bits in stained glass.
I've went in and filled in the area surrounding the image with black to better see how the image will look once carved. Doing that erased some of the details, so I've had to go back in and put in stray lines of White to redefine the edges of his head and jacket. Also I've added a little bit of text.
After this, you just print out a copy of what you're achieved (you can invert the colors to save black ink when printing) at the right size for your pumpkin. Then use whatever carving guides you know/like to carve your pumpkin.
I usually tape the sucker to my pumpkin and using a toothpick (or one of those little poky tools that you can get in a pumpkin carving kit) outline the parts to be cut our or scrapped off with millions of tiny tiny dots. Remove the paper, then start cutting (or scrapping).
here are two examples from my last 2 haloweens. XD
Don't remember which picture I used for the Original, just a normal front facing one.
Also, if you don't want to make your own carving guide, feel free to use the one included here. ♥