Linux Kernals

My friend is completing a uni assignment and is trying to understand Linux Keals do, as my knowledge is limited I thought I would ask here as i'm sure their are many people here who understand them fully. The question is below if anyone can help with the understanding of keals and the bootloader.

In Linux after the bootloader hands over the the keel image that loads base hardware and decompressed. When the keel decompresses fully that takes over and sets up the rest of the system, drivers, hardware etc...My question is, is the keel image literally just a zipped file stores to speed up the process and allow the main keel to decompress?Why is the image needed, couldn't the main unzipped / uncompressed keel just be loaded without?