Add background image using php

[admin]

I'm using wordpress and ACF plugins to add an image :

<a href="http://www.advancedcustomfields.com/resources/image/" rel="nofollow">http://www.advancedcustomfields.com/resources/image/</a>

I would like to add an image in a div as a background.
I used this code in my style.css but it doesn't work :

background-image: url(<?php $image = get_field('image_projet');?> <img 

src="<?php echo $image['url']; ?>" alt="<?php echo $image['alt']; ?>" />);

Thanks for you help