I just discovered, that the Nextcloud Android App and Files and Photos App of Nextcloud (Browser) are only showing a generated preview of the original image.
How can I view the original image?
Is the only way to download them to my device via sync or export?
I’m on apple but I expect it’s the same for you, the Nextcloud thumbnail/preview settings dictate the thumbnail size on the app, when you open the file you will see a full size version of the thumbnail (which is useless) until the app has fetched the full image file and displays that instead. I find it best to make everything available offline so that all the full images reside in the app so they display properly immediately and not have to wait for a download.
Side note: for this reason I set my Nextcloud previews to tiny 240px 60% quality because default setting is too high and disk usage is doubled by storing lots of low res images. Default previews are basically copies of the original, it’s stupid.