JPEG is designed to compress any image. But imagine a new algorithm, gJPEG, which is only designed to compress photographs of grass. And furthermore, you get 100MB of raw buffer space in the executable to store some precomputed data that would be useful to gJPEG doing its work. It's quite possible you could significantly improve on the general performance by factoring out some data that's common to typical grass photographs, so that data could be stored once-and-for-all in the decoder and then omitted from each of your (presumably) billions of individual grass photographs. On the other hand, it's pretty tricky to make it work, so you might not be able to do such a thing effectively.