BTW - I just tried to create a collage using photos sent to me by my mother from her Samsung Galaxy 10. No issues creating the collage.
So I used to be able to create a collage with my iPhone photos, but I no longer can. But I can create a collage with photos from another source, like a Samsung Galaxy...
I have spent weeks trying to get the photo mounting for collage working on my iPhone XS with Samsung to no avail. The latest is that I need to buy a Samsung phone because it is a known problem that the iPhone apps (SmartThing and SmartView) don't provide the same functionality as the Android/Samsung phones. I was able to get single large pictures using 3840 x 2160 using no border or Shadow mount but not modern mount (it shows the mount but with a black screed - not my photo). At no stage do I get to see the Collage - Matt option. Neither do I see the ability to select photos from my iPhone - I only got it to work at all by using the USB connection. Otherwise a great TV but Samsung have not delivered the promised functionality. The Samsung Support call is still open.
I think I found one reason, but maybe not overall root cause of the issues for pictures not uploading.
For my issue, I could only get some pictures to upload and others would either crash the app or give me the error that Art Mode wasn't available.
After following other guidance and not having success, I hypothesized it could be too big a file size or some kind of metadata on the file causing the error. So, I just opened the image on my iPhone and then took a screenshot of my screen. Cropped the image to the 16x9 resolution from other posts and sire enough it worked. The image had come from mermuseum.org free downloads. I suspect it was the metadata and not file size because I successfully uploaded other pictures of larger file size.
Hope that helps others.
I can confirm this is the fix that worked.
it is likely the size of the iPhone picture that is causing the issue or perhaps the fact that it may be a Live Photo from iPhone
After taking a screen shot of the photo already saved on my iPhone I now no longer get any error messages when trying to create a collage.
I haven't tried turning off my "Live Photo" to see if that may also solve the problem but I don't see any quality loss in picture when I took the screen shot so I can confirm everything works with an iPhone 11 and the 2019 Frame tv