Upload the target image to Content Management > CSS Override

Once the image has been uploaded you can reference it in the CSS. To find the address you can right-click the uploaded image and select 'Copy Link Address'/'Copy Image Location'/'Properties > Image Address'. Then simply paste that address into the code below (see addmelogo1.png example continued below) and add to your CSS file, upload and save configuration.  

.loadingArea .loadingLogo, .loadingScreen .loadingLogo {
width: 100%;
height: 100%;
background: url('{images}/-1241708800/Handler/CSSOverride/GetImage/39/addmelogo1.png') no-repeat center center transparent;
}