It works, you just need to give it permission to access your folders. It's macOS trying to defend you.
When you install it — don't try to open images with it, just run the ap first. It will show you what to do next.
This app is, by far, the worst thing I have ever tried. It utterly fails at its core job, which is to view image files! I threw half a dozen jpegs at it, all of which are perfectly fine and viewable in literally any other app, but Crossbow just drew a blank. You had one job here, Crossbow, just one, and you couldn't do it!