To export these as real image files:
Open this file in Chrome → Right-click on the banner → "Save image as" won't work for HTML.
Best option: Open Chrome DevTools → find the element → use a screenshot tool or Ctrl+Shift+P → "Capture node screenshot" in DevTools to export the banner at full quality.

YouTube specs: Banner = 2560×1440px (safe zone 1235×338px center) · Channel icon = 800×800px circle