Basically Bootstrap and Boot image are terms used for one and the same....
Boot image is actually a subset of the Cisco IOS (with no routing capabilities). ITs stored in ROM, on low end routers, while in Boot flash , on high end routers. Boot image version is shown by the "System Bootstrap version" command when the router boots up.
Hope that helps!
Let me put it this way.
Reading more on the fast boot option, i do see a referring to a link, categorizing, Main System Image, System Bootstrap image, and boot image as all separate entities.
Reading more on this, I agree that the above statement is true.
Bootstrap image - The first image loaded in a routers boot cycle is the Bootstrap image. This image is stored in a ROM chip. A BS image takes care of CPU and IO controller initialization. This image is nothing but the ROMMON mode that you see if the config register settings are 0x2100 and you reload the router.
Boot image - this is the image stored in boot flash. Mainly used to download images to cisco routers. ( maintenance purposes only). This is nothing but the RXBOOT mode, that you see on the router when you set config-register to 0x2101 and reload the router.
Main system image - this is the proper IOS that runs on the Cisco router.
Hope its now clear.
Fastboot - this is an option , that is available from, 11.2(x) i suppose. This feature speeds up the boot process, by using the system image(the main IOS) directly from the system bootstrap image, without accessing the boot image.