Hi, the images are placed in a folder on my server at root/photos/set1/ which I uploaded using Filezilla. It contains 5 jpg photos (0.jpg to 4.jpg) and 1 png image (0.png). See screengrab. These images/photos have simple numerical names i.e. 3.jpg. You would need to do the same on your server.
The 0.png (It is just a white rectangle with 70% transparency) is called by a placeholder which also tells the placeholder to "onmouseover="setnewimage(1)" or "onmouseover="setnewimage(2)" etc.
Your getting a 404 because doesn't exist. The n is changed to a number by Acorns code. Unlike my code which specifies the number of images, Acorns allows for an unlimited number of photos to be called from 0.jpg through and beyond 0.jpg to 12567.jpg
So the following code on a placeholder "onmouseover="setnewimage(3)" would change the base image from what it was to the following:
https://parkeston.com/photos/set1/3.jpg