I implemented a very reliable algorithm for this called Fast Multiresolution Image Querying. My (ancient, unmaintained) code for that is here.
What Fast Multiresolution Image Querying does is split the image into 3 pieces based on the YIQ colorspace (better for matching differences than RGB). Then the image is essentially compressed using a wavelet algorithm until only the most prominent features from each colorspace are available. These points are stored in a data structure. Query images go through the same process, and the prominent features in the query image are matched against those in the stored database. The more matches, the more likely the images are similar.
The algorithm is often used for "query by sketch" functionality. My software only allowed entering query images via URL, so there was no user interface. However, I found it worked exceptionally well for matching thumbnails to the large version of that image.
Much more impressive than my software is retrievr which lets you try out the FMIQ algorithm using Flickr images as the source. Very cool! Try it out via sketch or using a source image, and you can see how well it works.
