I would recommend considering moving away from just using an RGB histogram.
A better digest of your image can be obtained if you take a 2d Haar wavelet of the image (its a lot easier than it sounds, its just a lot of averaging and some square roots used to weight your coefficients) and just retain the k largest weighted coefficients in the wavelet as a sparse vector, normalize it, and save that to reduce its size. You should rescale R G and B using perceptual weights beforehand at least or I'd recommend switching to YIQ (or YCoCg, to avoid quantization noise) so you can sample chrominance information with reduced importance.
You can now use the dot product of two of these sparse normalized vectors as a measure of similarity. The image pairs with the largest dot products are going to be very similar in structure. This has the benefit of being slightly resistant to resizing, hue shifting and watermarking, and being really easy to implement and compact.
You can trade off storage and accuracy by increasing or decreasing k.
Sorting by a single numeric score is going to be intractable for this sort of classification problem. If you think about it it would require images to only be able to 'change' along one axis, but they don't. This is why you need a vector of features. In the Haar wavelet case its approximately where the sharpest discontinuities in the image occur. You can compute a distance between images pairwise, but since all you have is a distance metric a linear ordering has no way to express a 'triangle' of 3 images that are all equally distant. (i.e. think of an image that is all green, an image that is all red and an image that is all blue.)
That means that any real solution to your problem will need O(n^2) operations in the number of images you have. Whereas if it had been possible to linearize the measure, you could require just O(n log n), or O(n) if the measure was suitable for, say, a radix sort. That said, you don't need to spend O(n^2) since in practice you don't need to sift through the whole set, you just need to find the stuff thats nearer than some threshold. So by applying one of several techniques to partition your sparse vector space you can obtain much faster asymptotics for the 'finding me k of the images that are more similar than a given threshold' problem than naively comparing every image against every image, giving you what you likely need... if not precisely what you asked for.
In any event, I used this a few years ago to good effect personally when trying to minimize the number of different textures I was storing, but there has also been a lot of research noise in this space showing its efficacy (and in this case comparing it to a more sophisticated form of histogram classification):
http://www.cs.princeton.edu/cass/papers/spam_ceas07.pdf
If you need better accuracy in detection, the minHash and tf-idf algorithms can be used with the Haar wavelet (or the histogram) to deal with edits more robustly:
http://cmp.felk.cvut.cz/~chum/papers/chum_bmvc08.pdf
Finally, Stanford has an image search based on a more exotic variant of this kind of approach, based on doing more feature extraction from the wavelets to find rotated or scaled sections of images, etc, but that probably goes way beyond the amount of work you'd want to do.
http://wang14.ist.psu.edu/cgi-bin/zwang/regionsearch_show.cgi
