create image of a drive without unallocated partition.txt

The [count] and [skip] parameters scale with the [bs] parameter the default bs value is 512 bytes 
you can use 1K or 1M, but it uses the binary units 
and use [iflag=fullblock] to copy the fullblocks
The skipped size is [bs * skip]
The output size is [bs * count]

[fullblock] and [sync] are mandatory to be used
[skip] is optional, it is only used when you want to image a partition in the middle of the disk

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

2-open gnome disks to get the device block of the mounted image. it is usually like this   /dev/loop1

3-delete any unnecessary files to make the disk image smaller. After you finished deleting the files, unmount the partition so gparted resize it

4-sudo gparted [device block from step 2]

5-resize partitions using gparted

6-The sector count starts from 0.
So if the unallocated space is at the end. You will need to added 1 to the sector count. If you are using the default bs that is 512 bytes, you just need to add 1 without the multiplication because it is the default 512 bytes
(sector count+1)
count = (last sector in gparted) +1

but if the bs is not the default 512 value, the number of count or skip Wil be 
(sector count+1)*non default bs

if you are copying a partition in the middle of the disk, you do not need to add 1 to the sector count for [skip]
skip= first sector in gparted
count = (last sector in gparted) +1
if=[Can be the block device of the mounted image, the same one you used with gparted]

dd if=/dev/sdb iflag=fullblock of=disk.img count=1000000 conv=sync status=progress



Or alternatively you can

1- write the image to disk using gnome disks
2- update the files
3- run sudo fdisk -l /dev/sdx
4-sudo dd if=/dev/sdx iflag=fullblock of=/path/my_image.img count=From the end sector from fdisk +1 conv=sync status=progress


dd if=/dev/sdb iflag=fullblock of=disk.img count=1000000 conv=sync status=progress

dd if=/dev/sdb iflag=fullblock of=disk.img skip=12345 count=1000000 conv=sync status=progress

5-write my_image.img to disk using gnome disks
6- use gparted to fix gpt Partition Table
7-Create an image from the fixed partition