Chemically sensitized material (in this case fabric) is sensitive to the sun or any kind of UV light.  Sensitized fabric that is exposed to the sun will process and turn blue once developed. Whatever casts a shadow or blocks light from passing onto the fabric will prevent the fabric from processing, and that area will remain white. In the case of an acetate negative (a black and white photo, with the colors inverted, printed on to a sheet of clear thin plastic) the parts of the image that are a dense black will remain white in the final print, what is grey will show up as a light blue, and the parts of the image that are light (have no ink printed on the acetate) will process to a deep blue in the final print. Possible alternatives to using photographs; using literally anything that will prevent or obscure the sunlight from passing onto the sensitized fabric, this can be something like a leaf or a pair of scissors or a drawing on a sheet of plexiglass, pane of glass, etc.