As TaeSoo and nrapallo mentioned, you can change the output resolution by modifying two lines of 'config.lua'. You can change the first two lines to these.
Better yet, you can convert set of image files into LRF using comic2lrf of Calibre. First, make one zip file from the pictures then use a simple script such as
comic2lrf -c 8 -r 'file.name.here.zip'
I think it will be easier to manage one LRF file instead of hundreds of images.