Funny how you just summed up why the US plunged the whole wolrd into a crisis. 18% return actually *is* crazy huge. 2% or 5% (or whatever your average inflation rate might be) would be "not bad".

Exploit JPEG's weakness.

JPEG encodes pixels by using a cosine transform on 8x8 pixel blocks. The most perceptually visible artifacts (and the artifacts most suceptible to cause troble to machine vision algorithms) appear on block boundaries.

Short answer:
a. 2D-FFT your image
b. Use the value of the 8-pixel period response in X and Y direction as your quality metric. The higher, the worse the quality.

This is a crude 1st approximation but works.

Every single JPEG is lossy, for three reasons:

a. Source (color) digital images use RGB colorspace (typically, the raw format is "RAW" with a Bayer layout). JPEG compresses three planes, with a YCrCb colorspace.
Due to colorspace conversion and quantization error, you lose information. That's called "lossy".
b. Even in lossless JPEG, each 64-pixel block is KR-transformed and quantized. Again, always lossy.
c. No free lunch.

Typically, even lossless JPEG makes you lose 1-2% of the total information (measured via image entropy). Things are slightly better with lossless JPEG2000. Both are *perceptually* lossless.

Side note, but anyways: in Europe, a law cannot be canceled by jurisprudence. Short explanation: we don't have a "common law" based legal system but a legislative/codified system, as opposed to the US and UK.