The spec for align-content says:

Applies to: block containers, multicol containers, flex containers, and grid containers

An image is not a block container, nor any of the other containers listed.

So, per those definitions, align-content:baseline does not apply to an <img> grid item.

However, the spec for Baseline Content-Alignment says for Grid items:

A grid item participates in first/last baseline content-alignment in its startmost/endmost row or column (whichever is parallel to its inline axis) and if its computed align-content is first baseline/last baseline.

A strict reading of the spec implies that Baseline Content-Alignment does not apply to an <img> grid item since the align-content property doesn't even apply to images. Is this is the intention of the spec? I see no particular reason why images, for example, shouldn't be allowed to participate in baseline alignment using align-content:baseline.