Check out example codes for "background isn't working if in separate css file". It will help you in understanding the concepts better.
Code Example 1
background-image: url(../picture-folder/your-picture.png) 0 0; //No quotes, two dots before "/" //Also make sure that your <img> in html has some transparent image as src
Code Example 2
background-image: url(nickcage.jpg); // No quotes around the file name so just write it down without "nickcage".jpg
Code Example 3
background-image: url(nickcage.jpg); // first problem may be missing quotation mark // Second may be wrong path // check the name of the file if there is any error // check whether you are using proper extension of the file
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