Best Practices for Providing UI Designs and Mockups to Spec-Kit Agent

I’m working on an app and have created some UI screen mockups. I’d like to understand:

1. At which stage of the development lifecycle should UI designs be provided?
2. What is the best practice for sharing these designs with the spec-kit agent, given that it only accepts textual input?

Currently, I have visual mockups of the screens. How can I effectively communicate these to the agent in a text-only format?

Any guidance or examples would be greatly appreciated!