Image inside <summary>
content:url('/path/to/file.png')
HTML
<details>
<summary>Click to expand dropdown</summary>
<div class="image-container">
<a href="https://google.com">
<span style="content:url('https://r7.hiclipart.com/path/133/147/693/arrow-bullet-computer-icons-bullet-holes-e2beb2b74def46ebed678b15bb0e8012.png');width:100px;height:100px"></span>
</a>
</div>
</details>