Within PowerApps I made it possible to upload photo's to a document library and write that URL back to a column in a SharePoint list called Photo. The field type of the column is a hyperlink.
When a picture is used from the document library it's being displayed only in the PowerApps studio and not in the PowerApps Mobile App. However when a picture from a different (external) site is used it's showed in the studio and the PowerApps Mobile App.
It seems that pictures who need authentication on SharePoint are not supported yet to display within PowerApps Mobile App. I'd like it when the function could be implemented that pictures stored in SharePoint can be viewed in the PowerApps Mobile App and not only in the PowerApps Studio.