This article show how to change favorit star into any image e.g.

Please use this CSS Code

.mh-thumbnail__featured i {
display: none!important;

.mh-thumbnail__featured {
background: url(!important;
background-repeat: no-repeat!important;
background-size: cover!important;

Change width (50px), height (50px) and image url ( to your own.

In this article you can find information how to add custom CSS to any WordPress Theme: How to add CSS code that is future update compatible