If you can't see your custom ribbon button icons after upgrading to Unified Interface, follow the steps below to get them back!
Icons need to be in svg format to be rendered in Unified Interface.
You can convert your existing images using an online converter very simply. I used this site (https://image.online-convert.com/convert-to-svg) to convert my jpeg and png images to svg very easily.
After converting the images to svg, create new web resources with type svg, and update the ribbon button for the use the new Web Resource.