How to modify the background-image for a "NewSkin"

Error: I would like to modify the wiki background and replace the background image. As the admin, I opened the active skin, highlighted it and the skin preview appeared. I uploaded a new background image and saved. The first time I did it, a message appeared to save a file called "api.json". It saved in my downloads folder. I'm not sure what this is and where I should put it. Could you please tell me how to modify the background-image for my "NewSkin" so that it pulls the new graphic?

Solution: The problem is browser related, it didn't work with the Internet Explorer,but it works with another broweser like Firefox.