Now Google Image Search's index updates in real-time for many pages, just like the Web Search index. A few minutes after publishing a post, I was really surprised to see that an image from the post was already indexed by Google.
A search for [cartoon] restricted to the past hour returns 41 images and not all of them are from news articles and blog posts. Google Image Search still doesn't index all the images as soon as the pages are indexed by Google, but the improvements are noticeable.