If you intend opening an image to be used as an Overlay Image (found under the File menu) or as a Showthrough image after copying the image (found under Edit), please use an uncompressed image file (such as .bmp). A compressed image file( such as .jpg) will most likely add some artifacts in the black or white areas, which, whilst showing as black or white on the Canvas are not pure black or white. Using a compressed file will probably result in 'spots' around the edges of the Overlay or Showthrough part of the image..
Of course, you can use this 'feature' as part of your image if you want a random spotty effect.
