Write to a mounted image

gnome-disk-image-mounter --writable file.img

OR

1- sudo fdisk -l disk.img
2- value of the offset is (disk start number) * 512
3- sudo mount -v -o offset=VALUE -t ext4 /path/to/image /mount/dir

sudo kpartx -a -v file.img
It is a decent alternative if gnome-disk-image-mounter does not exist for whatever reason. 
But the only thing it can do is to write to the disk image. 
It can not be used to shrink the disk image

only gnome-disk-image-mounter can be used to shrink disk images and modify it at the same time.

remember to unmount the partition after you finished your changes. If you don't do that, the disk image will be corrupted