Since I don’t know how a taxonomy image in Wordpress works, I can’t tell you what would be comparable.
What you could also do, however, is use a structure field with two fields: tag/text and image. Instead of using the tags field type.
Kirby doesn’t use a database. So you have to build taxonomies and what you store within them in a different way.
One option is to build taxonomies as pages. Then you can add an image and other information in that page itself. Another option is using a structure field (in your events page, as part of the site settings or in a separate taxonomies pages) for the same purpose. It really depends on your use case what is the best way to achieve this in Kirby.
If this is a conference with only 4 days and you want an image for each day, you could, of course, just add 4 image fields to the events overview page.
label: Icon for day 1
label: Icon for day 2