At the moment, setting a custom image will only apply to one image ID at a time. So far, the player character only has two image IDs: "PC" and "PC_NUDE", for default and nude variants, respectively; so you can only replace those with custom images. There are no reactive image overrides and the reactive feature is only for the game's mannequin (which uses vector graphics to morph between each body part's states).
Due to technical and logistical reasons, and when put against the number of possible body part variations, I don't think there will be any image replacements for a reactive image at this time. To implement such a feature, we would have to decide what the major point of change has to be and make the reactive image changes based on that, such as only changing based on the character's breast size (so one image is used as a base, then iterating on various breast sizes--though not to mention that not all players will be playing a character with changing breast sizes).
If more characteristics need to be compared, such as skin tone, ear types, etc., a paper doll system will need to be built, which won't be any less complex than the current mannequin system, but will require much more work to make sure the layers fit well with as many different combinations as possible. Add in new images for any new body part types and you can see how massive an undertaking this will be, only exponentially increasing the workload depending on the body parts that need updating. This is ultimately why the mannequin graphics were implemented instead, to be a happy medium where the player can see only certain body part changes and infer the rest, in where the text will do all the heavy lifting on actually describing the character (as this is a text game after all).
To entertain the idea, I would probably go with a new category called something like "Reactive Custom Image", build a new system that will allow the user to select what the reactive value is (it needs to be numerically based, so like lip size, breast size, tone, or thickness, etc.--colors and body part shape types would be too specific to check against), lock the progression to 3 or 5 steps (small, medium, large, etc.), and have the user toggle if the set will have a nude variant. This will allow generating anywhere between 3 to 15 image IDs the user can replace with their own custom images. This will also require overhauling a lot of the current checks for the player character's bust artwork. And after all that, the question would be if it'll be worth the effort to implement this feature and if any player would actually use it in practice.