For some reason, a lot of sites reference to tools that are not needed.
All you need to do to resize a qcow2 image is the following:
qemu-img resize <diskimg.qcow2> +15G
This increase the disk size by 15 Gigabyte.

Then you need to use gparted to resize the partitions you want (use a USB live boot if you don’t have a GUI installed).