Feature for exact matching of images
Make use of perceptual hashes. It is very fast to compute and is very lightweight, both in terms of memory and cpu consumption. They are represented by simple long integers and can be indexed using many types of data structures such as VP Trees:

http://www.phash.org/

If that doesn't work, you can extract SURF features, quantize them into visual words using a vocabulary tree or BOW and index these with a database structure such as inverted file index. Check out OpenCV. It has many of the tools I mentioned here.