Text placed directly on a photo

Forbidden: White text over an image, legible on the mockup's photo and not on the next one
Instead: Contrast still has to be met — 4.5:1 for text 18px and under, 3:1 for larger. A photo cannot guarantee it, because the photo changes.

What this rule looks like

Forbidden

Instead

Four workable treatments:

1.
Linear gradient overlay — dark, ~90% opacity at the text edge fading to 0% partway across. Preserves most of the image.
2.
Semi-transparent overlay — a flat dark layer at ~50% over the whole image.
3.
Blurred semi-transparent overlay — as above with a blur behind the text only.
4.
Solid background behind the text — the caption approach; most reliable, least subtle. A text shadow may reinforce any of these but never substitutes for one. Verify against the worst image the slot will ever hold, not the one in the mockup.