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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