An image in the content of the page is a background in the styling, so it has no place for alt text.
Google and people using a screen reader cannot tell what a background image shows. For decoration that is fine. For a case photo or a product, you lose the description.
Snaglist lists images in the content that come from inline styling or from attributes such as data-bg. Per page you see the first image and how many there are. This is a notice, not an error.
Snaglist reports this as a notice. It costs 5 percent of the page in the health score. Good to know, and sometimes a deliberate choice.
In the report for your client this appears under the heading "Images with no place for alt text". When everything is in order, the report says: "No background images in the content".
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