imagemagick preinstalled

[user641]

megapixels will still work without it - it will just produce tiff files and not jpeg.

AFAIK megapixels has no dependencies.

To get the best out of megapixels make sure you also have dcraw installed (converts raw to tiff) as well as libimage-exiftool-perl (which gets some sensible exif data into the jpeg).

ps - for completeness see Documentation here.

Thanks for the detailed answer! Very useful, now I understand it much better.