Omg this is incredible, it's so generous of you to provide this whole thing for free. Can't wait to try it!
Hey, sorry for the late reply and thanks for the kind words. There shouldn't be anything stopping you from switching them out. There are a few things that could cause problems however:
First, try to force recompile the project and see if that works.
Make sure the names on your new images are exactly the same as the ones provided. The images also needs to be png files, you can change this in bj_core.rpy if you want another file format. Let me know if you still have trouble :)