aj-gps: Unfortunately it looks like there will be no way to solve the "riddle" in the near future.. [:(]
I've picked up the cache box earlier today.
So, here is the solution:
The background of the image is a high-capacity 2D barcode called "Dataglyph". This technology was developed by the XEROX Palo Alto Research Center (PARC). Because there is a lot of redundant information in the barcode it's readable even if you write on it - or fill it up with geocaching symbols, formulas etc.
PARC used to provide a free online encoding/decoding web page that could be used to decode the image resulting in a few lines of text, the parking coordinates and the location of the final stage. Unfortunately PARC has licensed the dataglyph intellectual property rights to Microglyph Technology and no longer offers the website to decode these Dataglyphs. Microglyphs are interesting as well but unfortunately it doesn't seem like there are any public encoders/decoders available.
If you are interested in that technology have a look at your nearest Milka chocolate package - there is a microglyph code on it [;)]