Test suite: document compress-image.sh usage

wiki/src/contribute/release_process/test/automated_tests.mdwn

@@ -108,6 +108,15 @@ Scripts:
 * [[!tails_gitweb config/chroot_local-includes/usr/local/lib/tails-autotest-remote-shell]]
 * [[!tails_gitweb config/chroot_local-includes/lib/systemd/system/tails-autotest-remote-shell.service]]
 
+# Expected, reference images
+
+Expected images, aka. reference images, live in [[!tails_gitweb_dir
+features/images]].
+
+When adding or updating one such image, run the
+[[`compress-image.sh`|contribute/how/documentation/compress-image.sh]]
+script on it before committing your changes.
+
 # The art of writing new product test cases
 
 Writing new `@source` features, scenarios or steps should be pretty