What are the components of the flash image?

Well, this is a wide question, but I"ll try to give an answer.

The preloader on Mediatek SoCs is a piece of software that intermediates between the operation system and the actual system bootloader.

It offers additional features like the ability to boot from either the eMMC or NAND Flash; it offers the ability to read/write various parts of the memory (flash) via USB (that what we call flashing). Also, it is essential because it decide what a boot mode a device may perform.

So it is not recommended to flash the preloader simply because flashing a wrong preloader (even if the flash file is correct, preloader code may differ) may result to a permanent brick also known as a hard-brick, the device becomes a paperweight: no booting, no charging, no USB flashing and in some case not being detected by a computer.

Additional infos on Mediatek partitions can be found here

Hope this answers your question.