2 or 3 weeks ago I experimentally added 2 400kb images (html scaled) toward the top of my home page instead of their thumbnails. My header & logo image is tied up in the CSS of the page and was the last to load, taking forever. It really looked awful and I did experience a about a %30 drop in page views.
Google updated its cache of the page yesterday and this page is now pushed back a page or 2 or 3 or 4 in the SERPs for keywords that are either associated with these pictures or fall near them. Most keywords that were present on the page before this update are unaffected except for some that are also toward the top of the page near the images.