There's some example files here that might help; Emily on discord has created some image packs which might be good to look at as well.
Extra reqs is extra requirements, yeah.
Extra prefs are intended to be used for extra stuff that's not built in - i.e. the code is setup to take a hair colour value and uses it to create a check (along with applying weights on the check that hopefully make it feel good when transforming characters). Extra prefs can be added on top of this for other traits that aren't built in. All three (requirements, preferences and the built-in checks) are used to tally a value for how closely an image matches a character, with the highest value (best match) being used.
Checks work off the current traits of a character rather than self image, but you can create a check that looks at someone's self image instead (it's a check that looks at the self image trait, more or less) so it's technically possible to do that as well.