I got tired of manually converting files either for wallpapers or icons so I wrote this easily modifiable simple script to do just that:

  #!/bin/sh 
  echo Making icon set here for:
  for file in "$@"
  do
    filename=`basename $file`
    echo " $filename"
    ext=${filename##*.}
    basename=`echo $filename | sed "s:.$ext$::"`
    convert $file -resize 16x16 ${basename}_16x16.$ext
    convert $file -resize 32x32 ${basename}_32x32.$ext
    convert $file -resize 50x50 ${basename}_50x50.$ext
  done