After downloading the image and PGP Signature verify it's integrity with sha1sum
and gpg
$ echo '<SHA1SUM> archlinux-x.x.x-x86_64.iso' | sha1sum -c -
archlinux-x.x.x-x86_64.iso:OK
$ gpg --verify archlinux-x.x.x-x86_64.iso.sig archlinux-x.x.x-x86_64.iso
should yield good signature