It is possible to add leading or trailing text to an element by CSS as described here. So if you can find a Unicode character that can serve as an appropriate icon, you can have it. See also these examples. Another option is to use an ‘icon font’ like Webdings and Wingdings.
If you want to insert a pure graphical object, this could be a way to do it:
.fa-joplin {
/* Awesome Font file */
-webkit-mask: url("data:image/svg+xml;utf8,<svg viewBox='0 0 1536 1792' xmlns='http://www.w3.org/2000/svg'><path d='M373.834 128C168.227 128 0 296.223 0 501.834v788.336C0 1495.778 168.227 1664 373.834 1664h788.336c205.608 0 373.83-168.222 373.83-373.83V501.834C1536 296.224 1367.778 128 1162.17 128zm397.222 205.431h417.424a7.132 7.132 0 0 1 7.132 7.133v132.552c0 4.461-3.619 8.073-8.077 8.073h-57.23c-24.168 0-43.768 19.338-44.284 43.374v2.377h-.017v136.191h-.053l-.466 509.375c-5.02 77.667-39.222 149.056-96.324 201.046-60.28 54.834-141.948 85.017-229.962 85.017-12.45 0-25.208-.61-37.907-1.785-92.157-8.682-181.494-48.601-251.662-112.438-71.99-65.517-117.147-150.03-127.164-238-11.226-98.763 23.42-192.783 95.045-257.937 81.99-74.637 198.185-101.768 316.613-75.704 5.574 1.227 9.55 6.282 9.55 11.997v199.52c-.199 2.625-1.481 6.599-8.183 2.896-.663-.365-1.194-.511-1.653-.531-21.987-10.587-45.159-17.57-68.559-19.916-.38-.04-.757-.124-1.138-.163-.537-.048-1.034-.033-1.556-.075-4.13-.354-8.183-.517-12.203-.58-.87-.011-1.771-.127-2.641-.127-.486 0-.951.05-1.437.057-1.464.011-2.886.115-4.33.163-2.76.102-5.497.211-8.182.448-.273.024-.547.07-.835.097-25.509 2.4-47.864 11.104-65.012 25.47-.954.802-1.974 1.53-2.9 2.36a1.34 1.34 0 0 1-.168.146c-23.96 21.8-34.881 53.872-30.726 90.316 4.62 40.737 26.94 81.156 62.841 113.823 35.908 32.67 80.335 52.977 125.113 57.186 35.118 3.36 66.547-3.919 89.899-20.461a97.255 97.255 0 0 0 9.365-7.501c2.925-2.661 5.569-5.5 8.086-8.416.3-.348.672-.673.975-1.024 8.253-9.864 14.222-21.067 17.996-33.148.639-2.034 1.051-4.148 1.564-6.227.381-1.563.81-3.106 1.112-4.693.555-2.784.923-5.632 1.253-8.49.086-.709.183-1.414.237-2.128.492-4.893.693-9.858.55-14.91h.013V521.623c-2.01-22.626-20.78-40.434-43.928-40.434h-57.23a8.071 8.071 0 0 1-8.077-8.073V340.564a7.132 7.132 0 0 1 7.136-7.133z'/></svg>");
}
I found this class styling in noteStyle.js, and in HTML it is applied like this in an internal link to another note “One-click sorting”:
<a ...>
<span class="resource-icon fa-joplin"></span>
"One-click sorting"
</a>
which shows a tiny Joplin icon before the note title. Regrettably I have no experience with this kind of graphical alchemy.