What I find more of an issue cognitively is that they entice to discern their contents, but of course they are too blurry to really see anything and trigger the subliminal feeling that you forgot to put your glasses on. So they attract your attention while typically not providing much useful information yet. A non-distracting neutral placeholder is generally preferable, IMO. Even more preferable would be for images to load instantly, as many websites somehow manage to do.