Board Thread:Support Requests - Getting Started/@comment-47145098-20201017131434/@comment-44807868-20201017132611

You don't need JS for that. It can be changed through Theme Designer. Under Background section there, you'd see a row called "header graphic"; that's what you're supposed to click on to choose the picture. Keep the limitations for the choice of picture in mind, though! "[...] For the header graphic, the same limits apply as for the backgound (.jpg, .gif, .png, maximum of 300 kilobytes). For optimal display, we recommend a file with the size of 471px x 115px. By default, it fades out to the middle of the page - keep that in mind when selecting an image!"

- Help:Theme Designer