first, if you don't have an excellent modern image editor, get one. GIMP might be okay, i've never used it. you can't go wrong with photoshop CS3 or later.
i like demonoid.com for backup copies of my software
then, you can pretty much use any image off the net with a few image tweaks.
in Photoshop, the magic tickets for deriving a dyeable image are:
- the Channels window... see the grayscale color separations of Reds, Greens, and Blues... often one of those channels contains the perfect black & white image. to copy one channel, select it in Channels, "select all", and copy. if the RGB/CMYK/composite is selected, you'll still be copying full-color.
the following > .. > .. paths refer to menus.
- Image > Adjustments > Black & White (new in PS CS3) ... this gives you control over how the image's colors are mixed for a B&W image. you can crank up the Reds to be white, while Yellows can become black. it's invaluable for creating the right black/white balance of light and shadow for your dye. the more brute-force techniques like Channels, Levels, Curves, and Threshold will yield less perfect results when used on a color image. i recommend this as the first step in prepping your color image for grayscale.
- Image > Adjustments > Levels .... lets you control the tone of lights and darks, set the midpoint, etc... helps to create a strong contrast between light and dark. i recommend this over Brightness/Contrast because you have more control over the spectrum.
- Image > Adjustments > Curves .... kind of like a Graphic EQ version of Levels... tweak a line on a graph to adjust the Leveling across the spectrum... gives you even more control, and you can get non-linear effects. for example you've got a face shadow, face skin tone, and white eyes and teeth. you could tweak the curve so that face shadow is just as bright as the whites, and leave skin tone as a gray. anything from shadow correction to psychedelic.
- Image > Adjustments > Threshold ... your very last step, after you've converted a color image to grays, and tweaked the separation of lights and darks. Threshold will set a split point where everything darker is black, and everything lighter is white. this purely black/white image is the reference you want to use when cutting your stencil.
- Image > Adjustments > Posterize ... plug in the number of colors, and it will average all your pixels into an image containing only that number of colors. great replacement for Posterize in a multi-graytone dye, or a multi-color if you're still working with a color image. experiment with Posterize on a prepped Gray and a Color image. try plugging in 6 colors for a 3-color dye, and you can decide yourself which colors to group as one.
- Adobe Illustrator has a great Auto-Trace feature that will derive a vector image from pixels. great if you want to go supersweet and start designing your stencils in vector format. after you trace, hit Expand and you've got editable shapes to tweak.
- Another Photoshop trick... once you have a prepped stencil image, if it doesn't come out looking correct to you, with everything meshing to the right overall optical effect, then just make a new layer on top and correct it. if the face's nose shadow bleeds to the lips and chin, then draw in some white to fix it. if the eyeballs disappeared or look like triangles, fix it. you can tell when a dye was stenciled from a straight set of algorithms, and when it was corrected by a human sensibility.
GOOD LUCK. try out all those techniques and processes, and become familiar with how they work. you probably won't use the same approach or sequence every time. and there are a ton of photoshop tutorials and message boards out there... google any problem you run into and you'll find plenty of answers.