Here are some insights:
* it's not important to get the top rankings, users often click on the next pages of results to find appropriate images. There are many "subjective" queries and users tend to explore instead of trying to find the perfect result.
* use images that are large enough.
* use high-quality images.
* include descriptive text next to the images.
* place the images so that it's not necessary to scroll too much in order to find them.
* Google Image Search clusters (almost) identical images and usually only one of them is displayed. If more than one page embeds the image, Google tries to find the most relevant page for that query.
* there are many new use cases for Image Search: inspiration, visual dictionary for foreign languages, shopping, research.