I neither use nor recommend exposing OH to the internet through a reverse proxy, so I don’t think I can offer a lot more in terms of advice.
If I were to guess, whatever subsystem on your phone that is displaying the notification is not authorized to access the page. You have basic auth enabled in the reverse proxy config and I don’t know if OH allows access to static without authorization too (I think it doesn’t).
Stuff like this is one of the major reasons myopenhab.org exists. Your iOS app will already be authorized to access that URL if you were using it instead of a reverse proxy and exposing your LAN to the internet.
And don’t you need myopenhab.org to do notifications in the first place? What’s the reverse proxy for? You already need myopenhab.org to deliver the notifications, so why not configure your iOS app to use it and then use the myopenhab address (https://home.myopenhab.org/static/FtpImage1.png) in the notification?
Why even expose yourself and take the risk of running and maintaining a reverse proxy in the first place?
Another approach would be to figure out how to get the image into an Image Item. See How to set the image content of an image item - #2 by Wolfgang_S for how to get started with that.