When you mean Illustrations do you mean still images or the expression / pose change stuff? Usually, each monster has its own Json file and within those files you have the image layers. You can create as many layers as you want, but you need to be mindful of the order in which you set them up. The top layer set will be applied first, so any layer that comes under that first layer will be applied on top. So you would have either the Nude or clothed layer first then the expressions layer under that, so they appear on top of the Nude or clothed layer.
As for complexity, it depends on how many parts the character has. Some like the Elf only have a single layer in which everything is tied to a single picture, so swapping them only requires a single Image swap, but more complex characters like Aiko who has a whole bunch of layers will require each layer to be swapped one by one as they are needed (Unless you create a layer set.) Illustrations would not be too hard to implement, as you can simply either add the image into the games image folder or make a Mod folder and set up the folders (See the Modding doc archive in the main game, it will take you to the modding wiki page that has everything you need.) Then on the Monster Json go down to the Pictures part and make a layer at the very bottom for the illustrations, making sure to include the file path for the image location. Then for any scenes including that character just change the image or apply the new layer to the illustration one and it will appear on top of all current layers. You can easily look at the existing Monster files to get an idea of how it works.
A little long-winded, but I am terrible at explaining stuff so apologies if it does not help. Alternatively, the Discord server has a few channels for Modding specifics which you can ask for advice in if you need more help.
