You can solve your problem using following steps

Step 1: Create a Javascript File and call a python function when document is ready.

$(document).ready(function () {
//Call a Python function 
//
});

Use what I explained you earlier  : https://www.odoo.com/forum/help-1/question/odoo-9-website-js-help-109603#answer-109604

Step 2: Return value of Image field in python function 

Step 3: Set Background image using JQuery.

Example

(new Model('your.model')).call('your_function').then(function (your_return_value) { 
     var image_format = 'url("data:image/gif;base64,'+ your_return_value +'")'
     //SET BACKGROUND IMAGE LIKE
     $(".oe_leftbar").css("background-image", image_format);
 });

All the Best  !