Need some help here, in my first attempts to make a flashable system image for our Jolla device.

I made a 'backup' of Jolla SailfishOS partition (mmcblk0p28=sailfish)

dd if=/dev/mmcblk0p28 of=/sdcard/system.img bs=4096

Now, I am trying to split that image into handable size using img2simg, like this

# img2simg -C 512 sdcard

But, apparently something is wrong with the block-size. I get this error:
""system.img" size (14761836032) is not a multiple of the block size (4096)."

Obviously, 14761836032 is not consistant with a bs of 4096.

I dont want an image of the entire /dev/mmvblk0 as I do not want to mess with bootloader, but want to accomplish an image that could be flashed like this

#fastboot flash sailfish sailfish.img

As some of you know, I want this, in case recivery gets screwed, and we are only able to get device into fastboot mode.

My bootloader is unlocked