We use ArcGIS Collector to inspect Trees which also involves collecting images as attachments. If an image is taken in portrait this still displays as landscape when viewing the attachment in Portal / Online. I would presume this is simply the default display by the browser rather than an AGOL or Portal issue, but is there anything that can be done?
We can work around this using web browser extensions to rotate the image but hopefully this will just be required as a temporary measure.
You may be running into the following bug:
BUG-000089912 : Pictures taken & attached in portrait mode from Collector for ArcGIS (android) for Samsung devices changes to landscape mode when viewed in Collector & ArcGIS Online.
We had the same issue using the iOs SDK in a native app. Our mobile developer was able to (add or remove I can't remember) a metadata tag that was forcing the rotate on other devices when displayed. I'll try and dig up the specifics. But I think if you compare the metadata tags from a iOs vs some other that tag should show up quickly.