If you need workflows, there's also VisualReview [1] with some basic workflow for approval testing, and Applitools [2], a commercial tool with integration with Selenium, Appium and Protractor that can do screenshot and video comparisons. I wrote about them as part of the 'Automated testing: back to the future' tools review [3]. In the article, I also mentioned DomReactor, but that seems to be discontinued now.
I wrote a tool that compares images using Applitools for Automattic during the React Wordpress admin rewrite a while back. The idea is you'd have a style guide with examples of all your ui components and render and compare them in various states with various test data. https://github.com/davidjnelson/css-visual-test
I have been researching visual review processes for a project[0] and never ran across VisualReview. I have mainly focused on the screenshot diffing algorithm, but planned on creating a nice notification/approval UX. It might be time for a little collaboration.