Scrum team- how does a designer function during sprints?

I've been thinking about this- how does a designer function in a cross functional, scrum team?

Lets say you have a sprint planning meeting on a Monday morning, and the team along with the PO decides to implement customer-facing features X and Y during the sprint.

Now X and Y require significant design work. While the designer is working on, say, making the PSD mockups for X, what will the front end engineer do until then? What if it takes the designer 3 days to do it? Will the front end engineer have nothing to do until then, assuming there is no work left from the previous sprints and there are no other bugs to resolve?