these scripts can be used as extension for gThumb image viewer for spliting the image into two halves, and convert them to grayscale. I am using this to convert my pdf notes to kindle friendly dimensions.
splits all images in a path Usage: ./split.sh path/
split selected images Usage: split.sh path/ filename.jpg
Tools->Personalize->new
split.sh
bash /path/to/split.sh %P
terminal command - yes
execute command for every file - no
wait for command to finish - yes
split_file.sh
bash /path/to/split_file.sh %P %N%E
terminal command - yes
execute command for every file - yes
wait for command to finish - yes