Thanks kovidgoyal!
MOBI is fine.
In regard of the image resolution, I was just hoping to get some best practice tips...
One more question:
I tested some of the output profiles.
What I find strange is, that if I use the "kindle output profile" the images look bigger in the "calibre viewer" than when I use the "tablet output profile".
Since I use images with 600px width and the "kindle output profile" is downsizing images to 525px, it should be the other way around...
Is there an explanation for that?