With the recent release of V2 of the Graph APIs, it is now possible to view thumbnail images of content stored in SharePoint document libraries within PowerApps (thank you!).
Equally this functionality is available regardless of the platform apps may have been launched on (i.e. both within the context of a web browsers and equally when launched from the native PowerApps mobile client apps).
Noteworthy references pertaining to this new functionality (including two of my own blogs subsequenty posted):
Given however that the overwhelming guidance for users insofar as uploading images to SharePoint(*currently at least) is to do so leveraging the Attachment control for SharePoint Lists data sources, how viable would it be to equally expose a ThumbnailUrl property for SharePoint List Item attachments?