I imagine it should be pretty easy with javascript to set up dummy images, and replace them... probably also doable in pure css, just make them a block element or something, with a set size...
You don't have to imagine everything. Normally the lazy-loaded image should be seeded with transparent SVG that as the same dimension. It's a solved problem.
I miss lowsrc. It had a point.
Wasn't this what progressive jpeg was invented for?