Whether this is necessary depends on your system configuration. An initrd image (the .img files to which you refer) is necessary if you wish to make available to the kernel some modules which is requires before it has loaded the normal root filesystem. For example (this is a silly hypothetical example), one might have ext3 filesystem support compiled as a module, but wish to use an ext3 root filesystem. In this case, there would be something of a chicken-and-egg problem, which would be solved by the creation of an initrd image containing the appropriate driver module.
Basically, try it without any initrd image referenced in grub, and if you have problems then you should know that initrd images are created using the "mkinitrd" tool.