Size your icon in advance.
Upload your custom icon to any documents section of Workfront (preferably somewhere that is accessble by you and other admins, an "admin project" or similar). View the details of that image and copy the url.
In your report column, under Advanced Options, add a rule indicating that you want to show icons.
Switch to text mode
Replace the /path of the pre-existing image with the path to your custom image and save.
You may need to adjust all of your rules before switching to text mode, if you plan on having several. Otherwise, you'll have to make all changes in text-mode going forward. Also, make sure you text this out as a normal user. I'm not sure if access level restrictions will prevent them from "viewing" your custom image without sharing it. If so, then just share that image with your companies or some other method and it should resolve it.