Hi Marko, I'll try to explain 100% JPEG compression better:
* All JPEG images are run through a process that breaks them up into squares of pixels (8x8, 16x16, varies based on quality) and then uses an algorithm to convert each set of pixels from pixel/spatial data into transform/frequency data.
* Converting from pixels to transform info discards some info. Even at 100% quality info is discarded.
* Each time a JPEG is re-saved, even at 100%, it's treated as pixel data & re-converted to a transform block-by-block. Thus, more info is discarded each time a 100% JPEG is re-saved.
So, 100% isn't just the best quality that WordPress can generate, it's the highest that the JPEG format can manage. And each time something is saved at 100% it gets worse. That's the concern here.
As for PNG, WordPress doesn't currently have any code in core for compressing PNGs. There are some libraries that handle PNG compression if you want to install them on your server. I discussed them in this support thread last year: http://wordpress.org/support/topic/png-optimisation?replies=13