In the meantime.
I helped myself and maybe this helps someone else so I write how I did it.
I had to erase in the base.css of the original theme the “width:auto” line under Images. Now it works.
Changing it just in the child theme’s style.css didn’t helped as it gets overwritten.
I just have to make sure the images have at least a minimum hight of 100px or the image overlay (circle with the arrows) doesn’t work anymore.

/* #Images
================================================== */

	img, a img {
		border:none;
		padding: 0;
		margin:0;
		display:inline-block;
		max-width: 100%;
		height:auto;
		width:auto;
		image-rendering: optimizeQuality;
	}

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