Add a background image to a content editor webpart.
/* You will need to use Dev tools to find the ID of the webpart and replace the #WePartWPQ3 with that ID */
#WebPartWPQ3 {
background-image: url(https://server/path to image/image_name.png);
/* path to image saved in ShPoint */
background-size: 100% 100%;
/* Sets the background image to the full size of the webpart */
background-repeat: no-repeat;
/* Only show one image, delete this line to show repeating */
padding: 5px 0px 0px 15px;
/* Add padding if needed to any of the sides of the webpart */
background-position: right top;
/* move background image to right top of the wp, to center delete this line */
}