The black background on the images appears only on those images that originally had a transparent background (in PNG and GIF formats), and after you have activated the Watermark module and rebuilt the thumnails. If you want to use the Watermark module for your product pictures you must convert transparent color into white color. This can be achieved easily by saving the PNG or GIF picture in another format, for example JPG format. This will automatically convert all areas with transparent color into white color. IrfanView by default is replacing transparent color with black color but this behaviour can be changed in the program settings.
I believe that the Watermark module should be upgraded to allow the Back Office administrator to select the color (black or white) to replace the transparent color in the PNG and GIF images while processing the images to superimpose the watermark.
