By default, llgal uses convert to generate thumbnails and scaled images
from your original images. You may change how the generation is done
in order to either:
  * replace convert with a specific image processing library that might be more
    optimized for your specific images.
  * apply some effects, for instance drop a shadow.

It may be achieved by changing the values of scaled_create_command and
thumbnail_create_command options in a configuration file. See the section
THUMBNAILS AND SCALED IMAGES GENERATION in the manpage for details.