A single photo
Under every tile in the grid there's a row of actions with a trash icon at the end. Clicking it asks you to confirm, then deletes the photo along with its preview and thumbnail.
All at once
In the bar above the grid, click More (···) and choose Delete all photos. The confirmation says plainly that this can't be undone, and the button in the window is Delete all. In a large gallery, deleting takes a few seconds.

What happens to your client's picks
A deleted photo disappears from your client's selection, so the Selected counter goes down. In an approved gallery, better not delete anything without telling your client: they'll see fewer photos than they approved.
Space comes back right away
Deleting a photo frees up space in your plan. It's the easiest way to get under your limit when uploads have been blocked.