The question asked is: Where is the "img" folder???
Currently, your css file is looking for the "img" folder in its css directory like this:
css/cssfile.css /* path to image: img/slogan.png */
Absolute vs Relative paths:
/img/slogan.png is an absolute path to the image.
img/slogan.png is a relative path to the image.
/img/slogan.png /* these folders are in the root directory */
/css/cssfile.css /* path to image: /img/slogan.png , or ../img/slogan.png */
img/slogan.png /* these folders are in the same directory as the html file, which may not be the root directory. */
css/cssfile.css /* within this css file, the path to the image may be different than in the above css file // path to image: ../img/slogan */