This has been driving me a bit nutz today.
Initially, I left the file location for this app unconfigured, but then @opentype's SuperGrid couldn't find the attachment because it was looking in my attachments directory.
I moved everything from the attachments directory to the uploads directory, and things worked. So, because I prefer to keep things tidy, I moved everything back to the attachments directory AND set the directory for Pages Category Images to the same attachments directory, and now neither this app nor Pages Supergrid points to the correct file.
I can upload a file to the category, but the preview is immediately broken. If I copy the URL to another tab but remove the attachments folder from the URL, the file loads, which means it uploaded correctly but not to the directory it is supposed to upload to. So, somewhere, somehow, this app is not respecting the file location settings under Storage settings in ACP.
My site is 20+ years old, so I really don't like dumping everything in Uploads.