I purchased a cs4 and cs5 in the past and I want to use one license on a mac but I can't find a suitable download image
Was the old license for Windows?
If yes then you cant do it because you may get the installer media but the serial number are also platform dependent , which will not work.
What Manish said. Serials are platform specific, so your Windows licenses are of no use. If you misplaced your install media and need to re-download an older version, try this:
Everything else will require an upgrade to CS6.