How to add a background image to Joomla 3 protostar template.

Go to ExtentionsTemplate Manageron left side you get

Styles

Templates

Choose Templates

Click Protostar details and Files→ go to CSS folder which is on the left side →  Click Template.css

Go to line 191 add background url.

body {
margin: 0;
font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
font-size: 13px;
line-height: 18px;
color: #333;
background-color: #fff;
background: url(http://www.mywebsite.com/images/wave.jpg) no-repeat ;
}

You should have gone to media manager and uploaded your image there.