Sketch referenced in this tutorial: Embedding Figma Files
That said, manually linking mockups can be cumbersome—you have to export new images from your visual design tool every time something changes in the UI.
A lot of folks are using Figma these days. One of the nice things about Figma is that it runs in the browser—which means that designs are accessible via URL. This video covers how to hook up frames from Figma to states in your spec.