Thanks to the GPG seal published by the Tails project, you will be absolutely sure that the image file thus created is identical to the image published by the Tails project.
Pre-requisites: you MUST have an ISO image file of some previous version of Tails, and also an executable xdelta3.
Details: you will download a VCDIFF patchfile, created by me (using the 'xdelta3' software), which describes the differences between the old version of the Tails image you already have, and the new one.
Usage:
export Tails_PREVIOUS_VERSION=6.11 # replace '6.11' by the version number of your image
export Tails_VERSION=6.12 # replace '6.12' by the version number of the newest version
xdelta3 -d -stails-amd64-${Tails_PREVIOUS_VERSION}.iso
tails-amd64-${Tails_PREVIOUS_VERSION}.iso_to_${Tails_VERSION}.xdelta3 tails-amd64-${Tails_VERSION}.iso
gpg --verify tails-amd64-${Tails_VERSION}.iso.sig
. If it does NOT produce a gpg: Good signature from "Tails developers (offline long-term identity key)" message then something failed (it never happened to me nor to numerous friends) and DO NOT USE the new image.