The main IOS will compatible with any boot loader image which can boot the rooter but best option is to keep them in same version.
Regarding boot load image, here is the explanation:
"The boot helper (boot loader) image resides in Flash memory and contains a subset of the Cisco IOS software. This image is used to boot your router from the network or to load Cisco IOS images onto the router. This image is also used if the system cannot find a valid system image.
Your boot helper (boot loader) image should correspond to the Cisco IOS release that is running onyour router.