convert

  convert                           \
  converted_source.jpg               \
 -write mpr:copy_source \
 +delete                          \
  mpr:copy_source -resize 100%  -write sample-1024x768.jpg +delete \
  mpr:copy_source -resize 66%   -write sample-800x600.jpg  +delete \
  mpr:copy_source -resize 33%   -write sample-400x300.jpg  +delete

convert.im6: missing an image filename `+delete' @ error/convert.c/ConvertImageCommand/3046.