Product Image Alt Banner
The Product Image Alt Banner shows a short text banner on single product gallery images. The text comes from each image's Alt Text.

To enable it, go to Site Settings in the WordPress left-hand menu and click Site Settings.

In the General tab, turn on Enable Product Image Alt Banner.

Click Update on the right-hand side to save your changes.
Then edit the product image in the Media Library and enter the text you want in the Alt Text field.

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You can edit image Alt Text from the Media Library.
If an image has no Alt Text, no banner is shown for that image.