Google is a great source to search for images. But sometimes when you are searching for a particular image, you find an image that is related to what you are looking for but not exactly the one you are looking for.
However, Googles new labs feature called similar images, which you can find at similar-images.googlelabs.com, makes it easy for you to find the image you are looking for. Just type in your search keyword and load the results. You will find several results matching your search term.
Now, locate an image that is close enough to the one you have in mind and then click on the Similar images link at the bottom of the image. You will get a whole new page of results with images that resemble the original image you selected.
For instance, if your original image is a night shot of the golden gate bridge, you will end up with a bunch of similar images from which you can make your pick.