You didn't include your HTML code in your question. Here a sample code showing this.
<h1>DIV with Background Image</h1>
<p>This div has an image as the background!</p>
Use below URL to paste above code & see output.
W3schools - Tryit Editor v3.6
In above sample, I have used absolute path to show you output. You need to use relative path for image.
Quote:joe.png is the name of the file and its in the same folder of css text editor
This part might be the issue. background-image: url('joe.png');
will work only if the HTML page(not CSS text editor) there in same path.
Check below pages for learning about File paths & update your code
HTML File Paths
Quick Reminder About File Paths | CSS-Tricks
<img src="picture.jpg"> - The "picture.jpg" file is located in the same folder as the current page
<img src="images/picture.jpg"> - The "picture.jpg" file is located in the images folder in the current folder
<img src="/images/picture.jpg"> - The "picture.jpg" file is located in the images folder at the root of the current web
<img src="../picture.jpg"> - The "picture.jpg" file is located in the folder one level up from the current folder