[Jellby]

Does KOReader support "display: grid"? If so, one could try something like this (works nicely on Firefox):

@supports (display: grid) {
  .illustration {
    display: grid;
    grid-template: "image" minmax(0,max-content) "caption" min-content;
    height: 100vh;
    width: 100%;
    margin: 0 auto;
  }
  .illustration .image {
    grid-area: image;
  }
  .illustration .caption {
    grid-area: caption;
    font-size: 80%;
    padding-top: 0.5em;
  }
}

<div class="illustration">
<span class="image"><span><img src="image.jpg" alt=""/></span></span>
<span class="caption">Blah blah blah</span>
</div>

(I'm not sure the <span> inside <span class="image"> is needed, but I must have it for a reason.)

The idea is that it should work with multi-line captions too, and the whole image+caption should take 100%.