I’m not sure exactly how late is late for you. A browser triggers image loading of actual image elements as soon as possible, even before having downloaded the whole page.
For images that are inside box backgrounds it triggers loading them as soon as they are noticed in the page CSS. Now because Sparkle has the CSS optimization (placing CSS for the top of the page directly inline in HTML), images for box backgrounds are triggered immediately as well. For box background images that are below the top of the page the loading is triggered as soon as the CSS for the rest of the page is loaded.
All of this is relatively quick though it of course depends on the connection speed, and notice I say “trigger loading”, loading performance depends on image size, on whether you are on http/1.1 or http2, etc. Many factors.