Jörg M Colberg [who blogs photography at concientious] introduces complexity and subjectivity with content-sensitive jpeg compression:
These Adaptive Jpegs (ajpegs)  – “American Pixels” – are an experiment. Jpegs are images where the original information was compressed to save space. A computer that creates a jpeg does not know anything about the contents of the image: It does what it is told, in a uniform manner across the image.
My idea was to create a variant that followed in the footsteps of what jpegs do, but to pharmaciefr have the final result depend on the original image: the computer algorithm becomes part of the image creation, in a very direct way. The idea was to build a hierarchical jpeg algorithm, where the compression – in effect the pixel size – depends on the information in each uncompressed pixel and its neighbours. So ajpeg is a new image compression algorithm where the focus is not on making its compression efficient but, rather, on making its result interesting.
The info of interest in many of Colberg’s images is military [they’re titled “American Pixels,” after all] but I like the way the variegated pixels play out in the more ambiguous, atmospheric images best.
 UPDATE: Jörg emails to say that because the term “ajpeg” has been causing some misunderstanding about the works and how they’re made, he’s changed it to “acomp,” short for “adaptive compression.” Duly noted. He’s also added some new images to his site; be sure to check back.
American Pixels [jmcolberg.com, thanks joerg]