It might be better to make a portrait/body generation system ( like the ones in RPGMaker games), it won't be hard to make standard-sized PNGs of various body parts and locate them correctly on the screen (and, maybe, automatically merging a new PNG). At least it's much better than just cutting portraits and figures out from some random pictures(of different styles!) downloaded from Pixiv (or wherever else).
I think it won't be hard for the author to realize this, since the game has already got its own nice portraits(like the helper, those merchants, the leader of the mage guild, etc).
Technically it is quite easy. Things needed: types of "body"(child&teen&adult and male&female and furries and types of butt, futas use female bodies and shota use some of loli bodies); types of breast; types of faces(child&teen&adult and male&female and furries, again futas use female faces, shota use some of loli faces), types of miscellaneous parts (ears, horns, wings, etc), types of arms and legs (mainly for furries and demons). The work of colouring can be given to several lines of codes. And finally, generate a new PNG portrait and a new PNG body for each "slave" character in savefiles (won't result in huuuuge savefiles, for we can always add a "click to update portrait" button, and use a silhouette pic for default, after all we have text descriptions). In this way, changes (say, clothing&gear, enlargements, mature/youth potion, pregnancy, etc) can all be shown in character figures.