@DaNi
The best software to do what you want is Image Studio. It will convert 24-bit iffs, jpegs, etc into HAM6, HAM8, with smaller palettes if required. You can also reduce the dimensions and a dozen other things. You can convert hundreds of images in batches without intervention. It's excellent.
Of course you need the registered version which has all these features unlocked. The demo version to be found on coverdisks, etc will not be suitable for your purpose.
Cheers,
JaX
