@crossingthethreshold The pool photo is a HEIC file, not a JPEG. So the Shortcut is probably uploading the original image, not the one from your conversion action.
When editing, tap Show More in the Get Contents of URL acition (the last one). There should be a Key called file there; make sure the value is the JPEG photo and not the original.
There is no such thing as a silly theory. 😊 But a weak WiFi signal results in either a slow connection (the Shortcut takes a while to run) or no connection at all. Shortcuts notice if you're offline, and you will get an error message: "The Internet connection appears to be offline."
The reason why you get the question marks is that you're uploading HEIC files. That's not a file format web browsers support. You can download the question mark file and look at it on your computer. So the Shortcut has been working all the time – it's just that browsers don't speak HEIC. cc @pratik