Is there any way to display an image as background? I have a picture in a field and I want to be displayed in its exact look but as a CSS background... is this even possible? Is there any mechanism how does that?
You can use Background Images and Background Images Formatter modules to achieve this. Note in the Background Images Formatter description that it can take images from a field and add it to the background.

Otherwise you can create your own module that generates the appropriate css and adds it to the page with drupal_add_css