What changes?
A photograph varies across its area, so one color pair may not represent the text background. A solid panel, sufficiently strong scrim, or quieter crop can stabilize contrast.
Use it in your work
Place a headline on a quiet area and check every responsive crop.
A common mistake
Relying on a small shadow to rescue text over a busy photo.
Context matters
The text contrast requirement still applies; keep essential text as real HTML.