Any time you place a file into Illustrator or open a raster image, you have the option to make the image a template. Templates are images on special layers that make it (slightly) easier to trace illustrations. The image used as a template can be black and white, grayscale, or color.
When the template image comes into Illustrator, it comes in on a new layer that is given special template status. A template layer is positioned under the current layer and set to not print. Any images on the template layer are ghosted (dimmed) to 50% of their regular shades, and the layer is locked so you can't inadvertently move the image. You can set all these options yourself by creating a new layer, placing the image, and setting all the layer options, but it's much easier to just place the image as a template. If you are the do-it-yourself type, see Chapter 8, where all the options for layers are fully covered.