What is a key difference between a zImage and bzImage kernel image?

zImage is compressed using gzip, bzImage is compressed using bzip2.

zImage is for 2.6 series kernels, bzImage is for 3.x series kernels.

zImage is limited to 64k, bzImage has no such restriction.

zImage gets loaded completely into low memory. bzImage will load into high memory once low memory is full.