How to add css depending on a field value? I have a field attached to a node which contains an image or not.

If there is an image I should load a specific css(sass). 

How can I do this?
Basically you need to drupal_add_css in your template_preprocess_node in your template.php after checking the condition that image exists in a manner similar to 

$field = field_get_items('node', $node, 'field_image');
if ($field) {
   //Do something with the field
}
else{
   //There are no results
}