Ok, I have found more on this. Before you load an image it shows the blue box (below) as placeholder. When you load an image, this seems to stay as on overlay.
But it depends on the image design:
Inline = it works ok
Poster, card, overlap, collage, stack = blue
By going into inspect, it goes away when toggling this off:
<div class="image-overlay" id="yui_3_17_2_1_1590120390667_457"></div>
Please fix!
Screen Recording 2020-05-22 at 2.03.15 pm.mov