Tell me, why do a screenshot if I can download apk and out to get all the pdf-file?
I don’t think this would work as image component in Thunkable X appears to have a fade in, but not fade out (I find this, maybe yours is different) and plus the images would have to be cached somehow and I have only noticed Android ever caching images, iOS never did this for me. So you would be working with internet connection and the photos having a white flash between switching photos. Not the best solution in my opinion.
You could watermark your stuff? Make a copyright picture with an invisible background and a few white stripes with your branding. Then absolutely position this over the photo.
In general both iOS and Android by now support the ability for apps to force-disable Screenshots. Android will just not let you make a screenshot and show an alert, in iOS certain parts of an app can be blocked/hidden in Screenshots - take Netflix e.g. - if you make a screenshot while playing a video it will appear just black where the video would be. However Thunkable would need to implement such a feature like Kodular did with their Screenshot component on Android. Maybe we’ll see such a component/feature one time but I think it might not be easy to do this the same on iOS and Android.
For now there is no such possibility in Thunkable X.