You'd better choose the Picture Column's type should be hyperlink, not picture, because app's image support url only.
I have tested if you choose picture style of hyperlink or picutre it will not shows in apps.
If the hyperlink comes from your Library, and you can also generate it to app, in the app's detail screen and change the card's type to image(you need to delete its cardvalue and then insert image )
And then image.f(x)=Parent.Default
Where do you retrieve these image files within your app? From your SP library?
Do you mean that the images from your SP library could be displayed well within your mobile app previously?
The user @Gidrone_NS has faced same issue with you, please check my response within the following thread:
Currently, Pictures that need authentication on SharePoint are not supported yet to display within PowerApps Mobile App.
If you would like this feature to be added in PowerApps, please submit an idea to PowerApps Ideas Forum:
If the Image files within your app are from your SP library, as an alternative solution, I agree with @xyzzyx666's thought almost.
You could consider create a SP list on your side, and add a Hyperlink column (Format url as Hyperlink) within it. Then copy URL link of the image files in your SP library, create corresponding record within your SP list and put the copied URL link within the Hyperlink column. After that, use the SP list as image data source within your app.
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