600x800 is *far* lower than the KA1's own resolution (which is something like 1404x1872 or something, IIRC).
The usual recommendation is to set your output profile to "Tablet", so at the very least Calibre won't mangle covers/images down to a ridiculous legacy resolution like that if the source is good enough

.
EDIT: Note that the Screensaver setting mentioned by @Ken Maltby specifically only concerns, well, screensaver handling, not inline covers/images

.