How to use timeline_item.add content field to use a repeating image Vis.js Timeline

I have been using the timeline_item add function with content field to represent the element with a image. It works fine.

timeline_items.add({
    id                        : entity_id,
    group                     : "timeline_group_id",
    start                     : start_date,
    end                       : end_date,
    content                   : "<img src='" + element_src_link + "'></img>",
    className                 : 'timeline-imagecontainer',
});

However, although I explicitly define a className, that does not help with the style properties of the image.

I want to have a repeating background with the img inside the timeline element.

How can I achieve this ?