It wasn’t long ago when Mikael Ainalem’s Pen demonstrated how you might use SVG outlines in HTML then lazyload the image (later turned into a webpack loader by Emil Tholin). It’s kind of like a skeleton screen, in that it gives the user a hint of what’s coming. Or the blur up technique, which loads a very small image blurrily blown up as the placeholder image.
José M. Pérez documents those, plus some more basic options (nothing, an image placeholder, or a solid color), and best of all, a very clever new idea using Primitive (of which there is a mac app and JavaScript version), which creates overlapping SVG shapes to use as the placeholder image. Probably a bit bigger-in-size than some of the other techniques, but still much smaller than a high res JPG!
