We can use HTML within the ‘content’ field of our tooltip item. Replace the url in the <img> tag with that of your image.
<script>
window.wmTooltipItems = [
{
href: "#tooltip-with-image-and-text",
content: `<img src="https://images.squarespace-cdn.com/content/v1/624b503d11269629b387b9c3/1649102910487-POB6XIE05ZPH03UR3ZG7/test-2.jpg?format=2500w"><p>This is my description</p>`,
},
]
</script>