Unable to send most photos via Messenger iOS App

As an additional note, I just tried with the Camera and this does appear to work as expected.  It's just most existing photos that seem to have an issue.

So the photo isn´t addad add all? Strange. No idea, sorry. Just digging in the dark. Which iOS Messenger app/M+ versions are you using? Have you searched for updates (I assume: yes, but to be sure...)? Can you add a screenshot (""Failed to Add Photo" ...)?

Are you sure the photo you selected is actually on your iPhone and not only available in iCloud? If it’s only in iCloud and not on the device it makes sense that Messenger shows an error because iOS cannot download the photo without internet access.

But wouldn´t that mean that the app is taking the photos first from the cloud and for sending via satelite (airplane mode or no cell wifi connection available) only if the photo is also available on the phone?

Well it is kind of an iOS thing, not a Messenger app thing. iOS can clear up space by deleting the local copies of photos (not sure after how long or when this exactly happens though) to free up space. So if any app then needs to access a photo that is only stored in iCloud, iOS will download the photo from iCloud before the app can do their thing with the photo. Depending on available free storage on your device, it could very well be that only a small subset of photos are actually stored locally.

So hence my question whether the specific photo the OP selected is actually stored locally on the phone or only in iCloud.

I did a quick test in the Messenger app with enabling airplane mode on my phone and adding an old photo from my Library (of which I am sure it is in iCloud only), and then I get the message "Failed to Add Photo". For photos which are locally on the device I get no error message and the photo is added to the message.

That's a very good point.  Let me play with that a bit more.  It's hard when, as far as I can tell/Google, Apple doesn't give you a way to know if an image is downloaded other than it being lower resolution when fullscreen and zoomed in.  You can see an icon for when it has been backed up, but I don't see anything for locally stored.

I was only selecting from my most recent photos to try to mitigate that, but after reading your post realized even my most recent ones may not be downloaded since I just upgraded my iPhone last Friday.  

If that is the case, it might be good to give the user a different error message (if that's possible to detect), or ideally attached the lower resolution photo/preview that is available on device. 

That was it.  I tested this way:

- Airplane mode ON, WiFi OFF
- In Garmin Messenger, attached an image that was old and I was sure wasn't downloaded -- I received the error as expected
- Opened Photos and confirmed that image was low resolution when zooming in
- Turned WiFI ON
- Saw the photo download in Photos
- Turned WiFi OFF
- In Garmin Messenger tried to attach the same photo again and it worked this time now that it's downloaded

Good that this is cleared. You had the right ideaPerhaps Garmin should modify the message which is shown in that case, that is more clear for the M+ user, what the problem is…