Go to the Overview page
Click on the edit button situated on the right side of the screen.
Add a Wild HTML Block in your Overview page and set the parent element to be “body” in the macro properties view.
The code used for replacing the background image can be seen below. Please make sure you replace the url with the one matching your image.
<style>
.btr-page-search-container{
background: url(http://clipart-library.com/img1/1212031.png) !important;
}
</style>
Press Publish and your picture should change.
Your new background should be available.