I have found the following thread: http://encode.su/threads/1250-ISO-im...?highlight=iso but i don't think it's exactly what I need...

I have these big ISO files coming from copying DVD (actually nothing illegal, I ACTUALLY just want to back them up . I've tried to compress them using simple stuff like zip and bzip2 but the compression was not worth it (10% smaller only). I tried paq8l and zpaq but somehow it fails on them (says "Nothing to compress"/cannot find the file).

Anyone has a suggestion? time matters less to me (anything less than 24h is fine) but size does. what's the best i can expect?