Fix the compress-image.sh script

cc @sajolida who might be interested in this change and in my commit messages.

For the curious, I landed there while working on #18963 (closed), and there I noticed that Tor folks use zopflipng (https://gitlab.torproject.org/tpo/tpa/team/-/wikis/service/blog#compress-png-files), and then I wanted to try it, which made me realize our current script did not actually use trimage (it did run it, but to no effect).