This effect was inspired by drawings of the countryside on Victorian teapots and teacups. Here are two small examples:
The dialog box is quite easy to use, but with enough to control to get just the effect you are looking for:
Normally the teacup is white china with blue cross hatching, and the default colors for this effect are naturally blue and white.
You can change the two colors to be whatever you want. You can also decide to have a frame if you want, and what shape and size that frame will be.
The "density" slider refers to the overall blueness of the final image. Generally it is best to have it at or near 0.5, but you may want to experiment with other settings, it all depends on what you like and your original image.