Including a large amount of raw data with a module
Your intuition is correct; you should break out your large dataset into some include file. I don't believe there's a strict mention of large datasets in the Drupal coding standards, but isolating out large datasets is good way to isolate concerns (i.e. code-readability vs. dataset referencing). 

A good reference example of this would be the Transliteration module. It has a transliteration dataset of include files indexed by unicode byte that's loaded on demand as it needs to transliterate a unicode characters to ASCII.

Your data will obviously be structured to fit your own use case, but given you're probably not the first person to work with large graphic library datasets and PHP, there's probably some general practices to glean from others who've worked on the same problem.