This seems to be a recurring bug... It was fixed previously but it's back.
When using a background image for the heading of each accordion blog, the background image does not show up.
This is because in /accordions/templates/accordion/accordion-hook.php, line 222 creates $header_bg_img with the 'url()' in it and then line 227 tries to show it by using esc_url_raw around it. It contains 'url(' it so the escaping always fails. The esc_url_raw function needs to be removed since it is already escaped in line 222.
We've fixed the issue on the latest version. Please install the latest version 2.2.90 and give it a try.