Hi there!

Thanks for contacting us.

Here is a sample code for you, need to be enterred in your custom CSS:

.event-list-item:after {
    background-image: url(localhost/rythm_quickstart_j4_v2.0.0/images/2022/01/01/_spmedia_thumbs/904232_393831067398232_2093578781_o.jpg)!important;
}

You have to upload the image in your media first then please use your image link in the above code.

Hope this helps!

Note: Path towards custom CSS(Joomla 4): System > Site Templates Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.

Best Regards