I'm no expert nor regular user of Rasterconverter - and would like to comment the following...
Because of the limited colours and resolution of the Atari graphics mode - if you start with pictures which are close to this already - you should have greater success - so to speak? And it is when you are using normal pictures/photographs as such - it is so much harder for any decent result to happen - without any of usual faults Rasterconverter ends up showing - those visible horizontal bands. People are only posting their successful conversions here - so you don't see the various failures that typically happen.
It is an amazing program - when you see what it can do - as in the various examples posted.
It is very much a lottery as to what picture ends up looking great - and usually you do see it pretty quickly - that you only have it running over a long period of time - just so as for it to clean up on some important detail here and there.
The good thing of course - is that you can have it doing this in the background - while you are doing other things on your laptop? And you can even have up to 3? windows of Rasterconverter running at the same time - if you wish to, if your laptop can handle it?