@EJJ, I cloned your experiment and I can tell you what’s going wrong. The file has it’s extension in lowercase (img_kitten_png.png), but in the image stimulus you refer to it with uppercase (img_kitten_png.PNG).
I assume that this worked locally because in Windows, filenames aren’t case-sensitive (i.e. Windows doesn’t care whether you write img_kitten_png.png or img_kitten_png.PNG). But on the Internet, this does matter.
By writing img_kitten_png.png instead, my version of your experiment did work online.