I\'ve got a div and apply :before and :after an image as content. That works perfectly. Now I would need to apply a background image so it does repeat
Background images on :before and :after elements should work. If you post an example I could probably tell you why it does not work in your case.
Here is an example: http://jsfiddle.net/namas/3/
You can specify the dimensions of the element in % by using background-size: 100% 100% (width / height), for example.