You can replace these images by navigating to the following element in the left menu:
Alternatively, to quickly see all the elements at once you can click the stacked arrow button at the top next to the word “Navigator”. This is how I found where the images were tucked away:
Finally, to actually make the change, click on the img element, then visit the settings in the right menu:
You will need to do this for each image by clicking on each appearance of img and swapping them out.
The script you are referencing, while related to the images, does not effect the actual replacement of the images. Let me know if you have any questions.