Shoebox indexes image content with algorithms that define shapes and colors. An example on its website shows a photo of a polar bear with a snowy background. Shoebox uses the bear's shape to find other polar bear pics. This isn't a perfect mechanism but if you are trying to sift through thousands of images for similar items, it's a great starting point. You can also add audio annotations to each image that are automatically converted to indexable text.
Shoebox is currently available as a free download.
If you're curious about future directions in image access, this website is a good place to visit.