> I doubt it would apply to PNG because of the length and content doesn't seem to be dictionary-friendly Correct - I wouldn't expect this to be useful for PNG.

> I doubt it would apply to PNG because of the length and content doesn't seem to be dictionary-friendly Correct - I wouldn't expect this to be useful for PNG. Compression dictionaries are applicable in situations where a group of documents contain shared patterns of literal content, like snippets of HTML. This is very uncommon in PNG image data, especially since any difference in compression settings, like the use of a different color palette, or different row filtering algorithms, will make the pattern unrecognizable.