Why not JPG ?

[alexaapetrei]

you could store meta data ( file name/s and type in the exif ) jpg essentially being a zip file anyway , also you could have a actual image of what the file represents, say a picture of the radiohead album or whatever :)

[ricardobeat]

PNGs are more like zip files than a JPG. The original idea was to store the data in the RGB values themselves, that would give it pretty good compression, but I ended up using tEXt because it was easier :) the tEXt chunks in PNG only support a limited range of characters; if metadata fields in JPEG can take arbitrary data you could put the raw or compressed binary in, otherwise it's just the same.