If the full images are large, and you have a number of them on the page (a gallery age for instance) some will remain empty boxes until others are completely loaded, as the browser limits the number of concurrent connections to each host.

IIRC the limit is most commonly 6. So if you have 7 or more images the 7th will not even partially download until the 1st has downloaded in full.

With this method, the small blurred images will load before any of the larger ones so you should see them all relatively immediately, absolutely immediately if encoded in the HTTP stream as data rather than being retrieved with an extra HTTP request, before any of the larger images start to transfer.