[fix] lazy loading of <img> tags

Images should include dimension attributes.  Without `width` and `height`
specified, image dimensions are 0×0 pixels at first.  ...  In this case the
browser determines that all of them are visible to the user and decides to load
everything [1].

In CSS the `width` is set to a value and the `height` is unsed to scale the image
proportional in both dimensions.

[1] https://web.dev/browser-level-image-lazy-loading/#images-should-include-dimension-attributes
[2] https://caniuse.com/loading-lazy-attr



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>