To resize the last partition in a disk image, for example a Raspberry Pi SD card image first increase the size of the file on disk using truncate, here we're going to increase it by 500MB.
truncate 2019-06-20-raspbian-buster-full.img --size=+500M
Once the file size has been increased the size of the second partition need to be updated to use all of the remaining space.
parted --script ./2019-06-20-raspbian-buster-full.img resizepart 2 100%
And then the filesystem will need to be extended to the size of the new partition, first enable access to the partitions in the disk image.
losetup -fP --show 2019-06-20-raspbian-buster-full.img
This will return the loop device name, in this example we're going to assume it's "/dev/loop1".
Then check the filesystem.
e2fsck -f /dev/loop1p2
Now we can do the actual filesystem resize.
If you see no errors then we're done and we can remove the loop interface.
losetup -d /dev/loop1
The size of the disk image has now been extended and the second partition has been increased to use all of the space.