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 !